The delivery of biological weapons is different from conventional weapons in that a lot of extra care has to be taken to ensure that the microorganisms do not get inactivated while being transported to the intended site of attack. 

Besides this, there is the problem of dispersion of the germs. Bioweapons , when utilized, should disperse the germs over a large area so that a large number of people are affected. Even if you are using an agent like smallpox which shows person to person transmission, it would be more destructive to have a large population infected initially, so that the "chain reaction" has a good head start.

It is for this reason that a brief explanation of how bioweapons are prepared for delivery is required:

Preparing your germs for the final journey:

Stabilizing the microorganisms :
This in simple terms means making the microorganisms less prone to inactivation. Not surprisingly, the techniques which are used widely in order to protect human vaccines find are utilized here.
Oragnisms may be preserved by freezing them, a process which is know technically as lyophilisation  . However, to make the germ even more "stable" a process called as Ultra-freezing is utilized. This process utilizes liquid nitrogen to create very low temperatures (-196° cent.). However, at such low temperatures, the liquids inside the germ cells (the cytoplasm etc) freeze, forming crystals which may damage the internal organelles of the germs, in effect killing them off. In order to prevent this, agents such as glycerol, dextran, mannitol, sorbitol, glucose, polyglycate , poly vinyl pyrillidone (PVP) are utilized.

Image : Oral Polio vaccine, which earlier was preserved by deep freezing 
 (-20°C, dry CO2). Recent vaccines are stabilized and can be stored at 4°C

A much more advanced technique whereby the microorganisms can be protected is by encapsulating the infective particles with a protective covering. Such encapsulation, however, is not necessary for weaponization. I have no technical details regarding this particular technique.

Ensuring dispersion of the Microorganisms:
The microorganisms are not distributed individually. Instead, they are grouped into "infective particles" which are generally 1-5 microns in size.

In general, the smaller the particles the better the dispersion. However, particles smaller than 1micron are counter productive since they are exhaled out by the human respiratory tract. In other words, particles less than 1micron do not STICK or are not retained in the lungs. Since the aim is to deposit the particles in the lungs and make the person sick, the "infective particles" in biological weapons are slightly more than 1-2microns in size. 

Explanation of the graph : Note the size of the particles that are maximally retained by various parts of the respiratory system.
Even though a particle size of slightly less than 1micron is maximally retained by the alveoli, particles of this size are not retained at all by the bronchioles, bronchi and the other parts of the upper resp. tract . Hence, in order to ensure the maximal deposition of particles in the respiratory system as a whole, particles around 2-5microns are ideal. 

In addition, the infective particles are charged with the same electrical charge, so that they repel one another (much like similar poles of two magnets repel one another). This leads to very rapid dispersion of these particles when they are let loose.

Image: Repulsion of bioparticles having like charges.

The following is an extract from "The Bioweaponeers" :
Suddenly he (Bill Patrick) extended his arm and heaved the contents of the jar into the air. His simulated brain-virus weapon blasted through the branches of a dogwood tree and took off in the wind, heading straight down a meadow and across the street, booming with celerity toward Frederick. Within seconds, the aerosol cloud had become invisible. But the particles were there, moving with the breeze at a steady ten to twelve miles an hour. 

The distribution of Death :

You just cannot fix the vials of your microorganisms on top of some RDX and blow the whole thing up so as to disperse the germs over a wide area. The efficacy of delivery of biological agents by explosive agents is estimated to be ~ 1-5%. Hence, more troublesome methods e.g. aerosol spraying (efficacy 40-60%) are employed.

The delivery vehicles may range from simple crop spraying planes to complex intercontinental missiles with refrigerated warheads. The former would be of use to the cash-starved bioterrorist while the latter is the preserve of nation-states engaged in this sort of research.

1) Aerosol Spraying :
    Although this is a very inefficient and dangerous way of spreading your bioweapons, for bioterrorists, it is the only way there is. An advantage of using this kind of delivery mechanism is that the agent does not need to be in a very highly advanced weaponised form. e.g. liquid anthrax , which is the more easily manufactured form of anthrax, can be dispersed in this manner. ( Only the US and the USSR are definitely known to have the capability to produce dispersible anthrax, the much more dangerous form.)

Images : Click on the images to view larger ones.
1) A crop plane of the type that is routinely used in the US.
2) I came across this advertisement while searching for "Crop spraying planes" in Google. Seems interesting. I hope the US authorities are monitoring these kinds of vehicles too.

Crop spraying planes are routinely used in the US for spraying pesticides over vast expanses of land. If you have seen one of these small planes in action (e.g. in the movie Independence Day), you have an idea about the large area a moderate sized sprayer would cover in a day.

Image : A handheld crop-sprayer. And no.. that Vietnamese guy isn't me.

Ground based sprayers may also be used for this purpose. This may be used in highly populated areas (eg. in theatres). Most of these things are very cheap to buy and can be bought in India itself (My family has a motorised sprayer for our garden). However, would anyone dare to be in the vicinity himself while spraying this stuff? Would anyone want to be sitting in a plane while his friend crashed it into a large building? The answer to both questions is yes.

News Flash : After the WTC bombings on Sept. 11, the US authoritites announced that stricter security checks would be carried out on all pilots operating crop spraying planes. The regulations for sale of such planes were also made much more stringent. ( Times of India ,   Sept 2001)

2) The more advanced systems :

These are usually in the form of missiles, or as parachuted bombs with little or no explosive to be dropped from planes.

The Russians employed missiles called MIRVs which had multiple warheads (up to 10) each targeted at a different location. The part of the missile where the germs were stored was refrigerated in order to protect them from the heat of reentry into the atmosphere. Parachutes are then used to slow down the descent of the warheads once they are released from the mother missile. The microorganisms to be used were presumably in a highly dispersible form (e.g. the Alibekov strain of Anthrax).

Image : Multiple warheads. These 3 warheads are of the Minuteman 3 , a US InterContinental Ballistic missile (ICBM).
These are nuclear warheads. The biological ones would, as already mentioned, need to be refrigerated.
No larger image.

The rest of the description is from "The Bioweaponeers" :
The biowarheads are parachuted over a city, and at a certain altitude (usually 50-100 meters above the ground - oncogen) they break apart. Out of each warhead bursts a spray of more than a hundred oval bomblets the size of small cantaloupes. The cantaloupes fly out a distance and then split in overlapping patterns, releasing a haze of bioparticles that quickly becomes invisible.

The Iraqis had admitted to having an arsenal of 16 missile warheads filled with botulinum, five with anthrax bacillus, and four with aflatoxin. They claimed to have destroyed these before the start of the Gulf war. However, a report by the US senate Research Service in April 1998 suspected that Iraq may still have hidden from 2-10 warheads designed to deliver chemical or biological agents.
It is interesting to note that Iraq presumably does not have the technical know how to produce the dispersible form of anthrax, which is usually used when such a delivery system is utilized.
In addition to the missiles, Iraq also acknowledged that it had manufactured 100 botulinum bombs, 50 anthrax bombs, and 7 aflatoxin bombs.

I do not have information regarding the US delivery systems for biological weapons.