Anger is seen from both the left and the right after today's opinion was made public today.  Stevens, Souter, Scalia, Ginsberg, and Kennedy voted to confirm while O'Connor, Thomas and Rhenquist dissented, making it a 6-3 decision.

The case came down to the question whether Congress had the right to regulate intrastate marijuana use, and do federal authorities have the right to arrest and jail violators in those states that approve it's use.

I think the latter was fairly obvious; if it's a federal law, then federal authorities have jurisdiction within any American state or territory.  Hence, really this case was only to reargue whether the marijuana bans by Congress was unconstitutional, something that was only argued about five years or so ago.  And if I'm not mistaking, Thomas actually sided for Congressional power in that instance.

Although Scalia's siding with the more leftist members of the court is a surprise to many, it really isn't.  There are a number of conservatives criticizing him for the overturning of a state law, but he's also a traditionalist.  Scalia isn't willing to overturn drug laws that have been on the books and upheld by every SCOTUS since the 30's.  I have no idea of what his exact reasoning is, but as far as I know he's upheld them all, and will continue to do so.  Call that activism if you like, as it might be a fair accessment.

As for the other supporters, it's real simple:  the legal justification of the marijuana bans were tied into the interstate commerce clause, which allows Congress to regulate all interstate commerce.  During the time of the New Deal, this clause has been used to justify a number of government programs, taxes, etc that allowed FDR's programs to be declared legal.  If those justices were to throw out Congress's marijuana laws, that would mean the interpretation of the clause would have to be reconsidered, and that's a no-no. 

The idea that Congress doesn't have the express right to regulate everything that's made a law would bring other Congressional regulations to question by that same judicial precedent.  So it's not surprising to me that the other supporters upheld it either.  It's a small negative to maintain a bigger positive.