Obama made many pre-election promises, one of which was that he would not federally prosecute people who were using marijuana legally under their own state laws. Of course, as soon as he was elected, as with all politicians, the promise vanished, and the DEA started raiding state-legal pot dispensaries. Now in the waning days of his administration, he is loosely following up on the federalism-respecting promise, as we see in this Huffington Post article: http://www.huffingtonpost.com/2009/10/19/new-medical-marijuana-pol_n_325426.html
We note that the rule is squishy, because it is left up to the discretion of prosecutors in the 14 states that have medical marijuana laws, so we can now imagine equal protection lawsuits across the Fruited Plain for something that should be a single, national rule. Indeed, the doctrine of federalism, if the president were really that knowledgeable about and devoted to the Constitution, would have prevented national government regulation of the area anyway, but I won't hold my breath until the Constitution is enforced in that arena, considering its manifold violations in so many others.
One question left open: the new "rule" [really a loose suggestion] seems confined to medical marijuana - what about the states poised to legalize recreational marijuana of small quantities?