Short answer: Don't hold your breath!
It could certainly be interesting though if Senator Obama offered the slot to Senator Jim Webb (D-Va.), who says the following in his new book: "The time has come to stop locking up people for mere possession and use of marijuana. It makes far more sense to take the money that would be saved by such a policy and use it for enforcement of gang-related activities."
Other then Webb, Governor Bill Richardson of New Mexico stands out for his yeoman's work on getting his state to become the 12th to allow medical marijuana access.
On the Republican side, the pickings are pretty slim although most people would probably be interested to know that although former Speaker of the House Newt Gingrich is no reformer on this issue, he has a very interesting history with medical marijuana and his own personal use in the 1970's when he admitted, "Smoking marijuana was a sign we were alive and in graduate school in that era."
Of the above listed, the safe bet says Senator Webb is probably the only one even being seriously considered for the gig. We will see.