Check out SCOTUSblog for slip opinions and analysis. This is, indeed, huge.
I'm hoping to find time to read the opinions this weekend and provide my own uninformed hare-brained analysis, but for now here's a quick observation. Based on the syllabus it seems like the Court could have stopped at the statutory interpretation holding -- that the military tribunals aren't authorized by Congress under current law, and are therefore illegal. The fact that they went on to consider the Geneva Convention issue at all, let alone determine that its provisions are judicially enforceable and applicable to the Al Qaeda conflict (reversing the D.C. Circuit on both points), is very surprising.