It sounds like a sci-fi cliché, but it is possible for one's cat to be radioactive, particularly after certain medical procedures - iodine treatment for a hyperthyroid condition is the most common.

If your cat has been pumped full of a radioactive isotope (like radioiodine), there are a few things to consider:

  1. You might want to kennel the cat for the first two weeks of the detoxification period, when the cat is the most radioactive. The cat's going to be miserable either way, but in a kennel you won't have to listen to the poor thing complain. It'll get over it.
  2. If you do bring it home and have other cats, keep them completely separated. The radioactive cat will be sweating iodine through its pores and allowing it to mingle with other cats is a bad idea.
  3. Be careful with the cat's litter box, as the animal's urine and feces will be radioactive. Clean it frequently, and check your area's regulations on the disposal of radioactive material. No, I'm not kidding - most places will forbid you to flush the waste and may require a special waste pick-up to take care of the stuff. Oh, and wear gloves.
  4. Don't go anywhere near the cat if you're pregnant. Just don't.
  5. Do exactly what your vet tells you to do - most local governments don't screw around when it comes to radioactivity and some pet owners have been hit with fairly hefty fines. Be careful.

Google (of course)
and a former co-worker's stories of his glowing cat.