Situation X: Radioactive Fallout Cloud. Population: YOU.

Unless it's absolutely impossible, your best bet is to leave ASAP and never come back if it's a nuke. Short lived radioisotopes and irradiated dirt are the least of your concerns. Strontium-90 has a half life of ~30 years, and is readily incorporated into the skeletal system if ingested. There's Iodine-131, which has already been mentioned, and has a half life of 8 days, and would remain a threat for roughly 2 months. But the worst is the bomb material that didn't undergo fission. If it was a Uranium bomb you will be relatively safe from it, since it has an incredibly long half life, the amount of radiation given off by it is relatively low. However, if it was a Plutonium bomb, you're in trouble. Not only is it radioactive, it's highly reactive and toxic in it's own right. The good news is that alpha particles can be stopped by a sheet of paper, the bad news is that there will be fallout everywhere, and some of it will be Plutonium.