With the passage by the state's of Colorado and Washington to legalize recreational use of marijuana in the 2012 general elections, this is likely to be a case in front of Supreme Court in the coming years. Only the Supreme Court will be able to rule on the Constitutionality of these state laws. Most likely, the question will come down to the viability of the Commerce Clause to allow the federal government to enforce the Controlled Substances Act (CSA). This is because creating and enforcing drug law is not one of the powers enumerated for Congress in the Constitution so the CSA is based on regulating the trafficking in marijuana across state lines and its impact on the national economy (something that Congress does have power to control via the Commerce Clause).
All we have to go on with respect to the Court's thinking on the Commerce Clause and its intersection with marijuana is the recent precedent in Gonzales v. Raich where the Court found broad powers for Congress to regulate marijuana because of its interstate effects. In this case, even the personal growth of marijuana for individual in-home medical use (as allowed by state law in California) could have negative impacts on interstate commerce and therefore could be regulated by Congress.
Importantly, the reasoning of the Court in the case started with the unchallenged presumption, conceded by both sides, that Congress had the authority to regulate marijuana for non-medical uses. The Court then went on to say that because marijuana grown for medical reasons could be used for non-medical reasons (perhaps the product was stolen and transported across state lines) it could also be banned by Congress to prevent that activity from occurring. From that decision:
The parallel concern making it appropriate to include marijuana grown
for home consumption in the CSA is the likelihood that the high demand
in the interstate market will draw such marijuana into that market.
Given that the Court took as a foundational assumption in Raich that non-medical use of marijuana could be banned and that above and beyond that so could medical marijuana, the likelihood of these laws standing up against a court challenge seems unlikely, but only time will tell for sure.
In practice today, however, the Obama administration has chosen to focus their efforts on what they call higher priority issues facing the Department of Justice (budgets and resources are limited after all) and have chosen thus far not to enforce the CSA violations in these states.