Will Obama Let Washington and Colorado Keep Their Legal Pot?

Congress could also help out by reclassifying marijuana so it's not a Schedule I drug, and/or amending federal drug law in a manner that provides for more state authority over the subject. Under current law, the DEA and Justice Department are legally supposed to enforce prohibition vigorously and not allow any usage, since Congress has decided that there is no such thing as legitimate usage, not even medical usage. Congress has also chosen to preempt state law on the subject, so the Justice Department doesn't formally have any authority to defer to state policies.

It's true that Obama could essentially look the other way and enforce that law only weakly: he could devote fewer resources to it, the Justice Department could vaguely cite logistical difficulties in coordinating with state law enforcement as a way to de-facto defer to states more, etc. Lobbying him to do that might be the only short-term solution with non-zero odds of success. But fixing the law would be better imo.

Federal prosecutors are fairly independent, so Obama has less control over there actions than you might think.

Given the directions from the Department of Justice and the statement made after the election and what they've done in CA, the answer is no. The President has been very consistent since taking office despite what was said on the campaign trail.