What does "Conspiracy is not a war crime" mean?

One issue about Hamdan that I have not seen discussed much is the plurality's ruling that conspiracy is not a war crime. Kennedy didn't join in this part, and it is arguably dicta. But it is slightly troubling, since it might make those involved in planning and giving logistical support to war crimes immune, at least if they didn't directly give orders or do the shooting. I'm not sure that this part of the decision will be followed internationally.