It doesn't mean they've decided to rule in favor of the Gitmo jailbirds - it might, but it might not.
Except when there is a disagreement between federal circuits on how to interpret a specific law, the Supreme Court almost always grants cert to reverse the lower appellate court.
Remember that Roberts had to recluse himself from the Hamdan case due to his prior involvement in the [Appellate?] stage of the case. I haven't been paying much attention as of late, so I was wondering if you happen to know if Roberts will be able to weigh in on this one? IMO, his reclusion from Hamdan is the only reason the plurality was the plurality. And the plurality, or majority, in that case willfully ignored so many facts and twisted their reasoning to such an incredible extent in Hamdan v Rumsfeld et. al., that, imo, that case and their finding seriously erodes the credibility of the Supreme Court, who are supposed to be above public opinion and, in particular, above the influence of insane and idiotic liberal movements. Perhaps, as you suggest, this is Roberts' attempt, as Chief Justice, to restore dignity and credibility to the Court. They need to do something! But then, this could be a move from the Left on the Court, in which case we are in an historic moment - the fall and disgrace of the US Supreme Court and the Judicial Branch of US governance.
The usual senile old suspects and professional poisoners hauled this one in. No big thing.