In short, the liberal wing of the court stuck with the notion that the feds can trump state laws and regulations on certain matters -- a stance that has allowed it to intervene in issues like civil rights, labor rights, and the environment.
Scalia and Kennedy, two supposed state-righters, joined the majority. Kennedy can be a bit erratic on such issues, so I'm not sure what was up with him. But Scalia? Is this decision any more evidence that he's a partisan hack with no respect for legal reasoning?
I have far more respect for Rehnquist, Thomas, and O'Connor, who held true to form despite opining on an issue that is anathema to conservatives (drugs).
In fact, I would venture to say that Thomas is perhaps the most consistent member of the court. I could say with 99.9 percent certainty how he will vote on most issues. Just interpret the Constitution literally.
As for the substance of the decision, it doesn't matter much. The Court said that states had no obligation to enforce federal laws, and the DEA isn't going to go around and arrest cancer patients growing weed in their homes. They don't have the manpower to do so, nor the will to generate that much bad publicity. It's a sort of "don't ask, don't tell" arrangement.
But the most interesting part of the decision, by far, is the Scalia vote. He's a partisan hack who lets his personal views on issues cloud his legal reasoning. He is a states righter one day, then demand federal supremacy the next. All depending on which is the best vehicle to promote his agenda. Just like the GOP at large. The fact that Bush would consider him for Chief Justice is probably apt.
Update: Meteor Blades links to this site listing DEA arrests of patients using marijuana for medicinal purposes. So it looks like contrary to public declarations, they ARE going after some of these people.