With the help of international law, the Supreme Court psychically communed with the American zeitgeist and discovered some sort of latent consensus that executing juvenile criminals is "cruel and unusual." Letting people suffer because pot is baaaad, mmkay? Apparently, in light of the 6-3 decision in the Raich case today, that's cool. It's also, mystifyingly (though, given Wickard v. Filburn, not at all surprisingly) the government's prerogative under the Interstate Commerce Clause, even when the medical marijuana in question is grown and consumed within the borders of one state.
Update: The opinions are here. Thomas lays it out pretty succinctly in line one of his dissent:
Respondents Diane Monson and Angel Raich use marijuana that has never been bought or sold, that has never crossed state lines, and that has had no demonstrable effect on the national market for marijuana. If Congress can regulate this under the Commerce Clause, then it can regulate virtually anything--and the Federal Government is no longer one of limited and enumerated powers.
Of course, he could've written precisely the same thing about Wickard. NORML has a response up with links to further information.