Can and will are two different questions, but it seems that if the Obama administration indeed seeks to overturn California's marijuana-legalization measure in court--as a handful of former DEA administrators have called for--the Department of Justice will have trouble doing so.
The law contains two broad parts, one that legalizes possession and growing for personal use and another that allows counties to set up commercial-licensing regimes for pot cultivation and sale. The former, it seems, would stand up to a challenge in federal court; the latter may not.
I took a look at the issue in the latest issue of National Journal (subscription required). Here's what two legal experts had to say:
"Constitutionally, California will have a valid law," said Ted Ruger, a professor of constitutional law at the University of Pennsylvania, who has reviewed both Prop 19 and Raich. "State officers don't enforce federal law and can't be made to enforce federal law." ...
"We're talking about the state attempting to authorize a system of regulation of commerce ... a form of regulation that is contrary to what the Controlled Substances Act contemplates," said Eric Sterling, a former House Judiciary Committee counsel who now is president of the Criminal Justice Policy Foundation, in addition to consulting for the Prop 19 campaign. A legal challenge to the proposition's commercial sections, he says, would not involve the federal "commandeering" of state law enforcers or judges and might succeed in federal court.
Pick up a copy of NJ to see the full story, including more on what, exactly, Prop. 19 does and background on of California's last big clash with the feds over marijuana policy.