Really want to clear out an area? Try making an anthrax threat. The following is the procedure in case of an anthrax threat (I believe it is pretty universal):

1. Building is evacuated. The place is basically sealed - no one, not even police enter.
2. The local FBI field office and the CDC (Center for Disease Control) are called and respective representatives arrive on the scene.
3. Everyone who may have even walked within 10 feet of the building are vaccinated (as many people as the police can find).
4. Tests in the building are taken for anthrax by the CDC.
5. The building is kept clear for another 3 days.

Obviously you gotta be extremely careful if you choose to do this instead of a conventional bomb threat since the FBI will be all over and will probably be pretty angry.

2005 update. Since this node was written (pre-911) larger cities have probably created much better policies for handling these kinds of threats. It likely won't work half as well as before and probably gets 2x the amount of pissed off homeland security people looking into it.