Most states simply require you to sign your driver's license, and it's probably a good idea to tell your family that you are a donor.
However, some states also have a donor registry, including New York and California, among others. To find out what your state requires, go to:
http://arago4.tn.utwente.nl/stonedead/movies/meaning-of-life/11-live-organ-transplants.html: Don't do it!