Well, first and foremost, I doubt that the feds would go after California too hard. President Obama, unless he wants to seem tough on crime or something like that, has bigger problems to deal with than someone smoking pot, so the prosecution/apprehension of smokers wouldn't be that strong.
Even if the Feds did decide to pursue, the AG of California, Jerry Brown, is going to go to a Federal Judge for a stay of action, which he will probably get, and Brown will also oppose the law as Governor. So while federal action is stayed, the Feds sue under the Federal Controlled Substances Act, and this gets bumped up to the Ninth Circuit. Eventually, around 2014, it's going to hit the Supreme Court.
Now, one of three results occurs here:
A) The Court strikes Prop 19 down-Unlikely.
B) The Court allows Prop 19 to go through-Likely
C) The Court declines to hear the case-Probable.
The reasoning for this is Gonzales v. Oregon, from a few years back. The Court ruled then that Oregon had the right to enact the Death with Dignity Law, and that the government couldn't go after doctors under the Controlled Substances Act.
While the Court has changed, it's not enough to overturn a precedent. The liberals are going to vote for California, and Kennedy will go with them, possibly Roberts or Alito as well.
So Prop 19 is legal.