I think he's going to wait until he's proven that he's true to his word as a leader on the issues he's made promises for. (removed)
I agree, as the USPOS he has a few other responsibilities he needs to act on first. He alone cannot change marijuana laws.
On the other hand, if Mr Frank's bill ever makes it out of committee and to the floor for a votes, and then to Mr Obama's desk then I'm pretty sure Obama would sign a bill that decriminalizes mj or at least mmj.