One of Hamdan's lawyers, writing for Slate, notes that there is no precedent for a conspiracy charge violating a law of war, yet this is one of the charges against all of the people facing military tribunals at Guantanamo Bay, and is the sole charge against seven out of ten of them, including Hamdam himself. He also notes that the government does agree that military tribunals are limited to prosecuting laws of war.
A ruling from the U.S. Supreme Court that this is not a violation of the laws of war, could effectively shut down most military tribunal cases and make those that remain, in theory, more difficult to establish.
Obviously, this is a fall back position. Hamdan would prefer a determination that the entire military tribunal system was unconstitutional, and that the Detainee Treatment Act, as a suspension of habeas corpus in a situation where the constitution does not provide for such a suspension, is also unconstitutional. But, it might provide room for a consensus on the court, if it can overcome the jurisdictional issues provided by the Detainee Treatment Act, on, at least, some issue.