well, along with signing up for the organ donor program on your license, it's important to let your family/next of kin know how you feel about this.
As far as I know, legally it's still the next of kin's decision what to do when you are brain dead (brain dead is legally dead btw, it is NOT a coma. there are very specific criteria, and it involves the loss of higher brain (cerebral) function as well as the "lower" brainstem function that sends messages to the lungs to breathe etc. thus there are not that many candidates to be organ donors).
i believe the little sticker or notice on your license is merely evidence of your desires--i.e., if i were brain dead and had the organ donor mark on my license, the transplant nurse/counselor assigned to my case could use that to persuade my hubby that it was my wish to give my organs. if there is no next of kin, i guess it is legally binding, but i'm not sure.
just thought i'd mention that