Kevin Drum and Mark Kleiman suggested that the president quietly allow Colorado and Washington to experiment with marijuana (hokey "experiment" usage intentional). This is a great idea, though I don't know if the president would agree.
So why shouldn’t the federal government cut Colorado and Washington some slack? As long as those states prevent marijuana grown under their laws from crossing state lines and thereby subverting marijuana prohibition in the rest of the states, the Justice Department could step back and let the consequences of the new policies play themselves out. They might succeed, or they might fail. In either case, the rest of us could learn from their experience.
If it works, and the states don't disintegrate into hellscapes of wacked out, snack-seeking zombies, and if, in fact, tax revenues increase, why not legalize it at the national level?