"Actually, the Medical Marijuana laws (and Colorado's new law) are not nullifying Federal law. They are a separate set of laws for handling drug cases that come up for prosecution at the state level (or instructing local law enforcement to ignore those cases entirely)."
That's what nullification *IS*. It is State refusal to obey, or cooperate with, Federal law.
"If the FBI picks you up for trafficking in those states, you're not going to state court, but federal court. And in those courts, I guarantee you that nullification will not be allowed as a defense."
You aren't getting it. Nullification is not a "defense" in court. That isn't the way it works.
"State nullification" refers to States refusing to comply with, cooperate with, or obey Federal laws that they believe to be unconstitutional. As in the example I gave earlier: Washington and Colorado legalizing marijuana for "recreational" use. This is very, very much against Federal law. The States are doing it anyway. That is a textbook example of "nullification".
YOU are referring to some kind of "official" vote of the states to formally somehow "nullify" Federal law. But that's not the way it has ever been done.
The main point here, however, is that while it might not be the "formal" process you envision, it IS done. Frequently. And successfully. And has been done that way (as I mentioned before) repeatedly and successfully for 200+ years.