The opinion is out in Boumediene v. Bush (opinion here in pdf format), and it appears to be a pretty thorough rebuke of the administration's position. The Court ruled 5–4 (with Justice Kennedy writing an opinion that the four liberals signed onto) that detainees at Guantánamo are entitled to bring habeas petitions in federal district court, and that the alternative set of procedures envisioned by the 2005 Detainee Treatment Act isn't an adequate substitute. If Congress wants to take away the detainees' habeas rights, it needs to do so in accordance with the Suspension Clause of Article I of the Constitution. Something tells me Patrick Leahy and John Conyers won't be very interested in doing that, but perhaps it'll become a campaign issue now.
See also the Associated Press and Lyle Denniston at SCOTUSBlog for more.
--Josh Patashnik