Finally. Look, everyone knew Gitmo was illegal. The whole "enemy combatants" (as distinct from PoWs) was always nonsense on stilts (with apologies to Bentham). The interesting thing about this is not that Dubya is going to close Gitmo -- he won't -- but that the Supremes have finally woken up to the fact that they are the third branch of the US government, and, if the legislature is asleep at the wheel, then it's up to them to check executive power. It also seems that this may have a negative effect on Dubya's ability to find laws that superficially justify his illegal wiretapping program (again, he won't stop doing it, he'll just look like more of a power-hungery idiot). My hope, of course, is that Bush will continue to defy the law, and will expose his party to utter defeat in mid-terms and in 2008.
The worrisome thing about the decision, of course, is that Thomas, Scalia, and Alito were the dissenting opinions. Roberts recused himself; even if he had weighed in, it still would, at worst, have been 5-4 (and, at best, 6-3). This means that conservative appointees think that the President of the US can do whatever he wants, as long as he follows his declarations with "Yah, boo, scary, terrorists under the bed!" Hence, the next vacancy must be filled by someone sane -- it's as simple as that.
Edit: As this makes clear, Gitmo itself is not illegal. It's the military commissions that Bush was trying to use instead of real trials. Fair enough -- but, if Gitmo detainees must be tried (as they must) and kangaroo courts are no longer legal, then Gitmo comes closer to being an actual (acceptable because necessary) prison instead of a gulag. Again, I expect Bush et al. to try to use military commissions anyway, or simply give up on even mock trials, but at least the Supremes are in the ballpark of doing their job.