Mark Kleiman thinks not:

The federal Controlled Substances Act makes it a felony to grow or sell cannabis....The non-medical cannabis industry that would be allowed if Proposition 19 passed would quickly fuel a national illicit market....As a result, pot dealers nationwide and from Canada, for that matter would flock to California to stock up. There's no way on earth the federal government is going to tolerate that. Instead, we'd see massive federal busts of California growers and retail dealers, no matter how legal their activity was under state law.

Kleiman still plans to vote for the proposition as a protest against current drug laws. Drum is more optimistic:

There's really no telling what the feds will do until someone forces the issue. So why not force it? At the very least it has a chance to move the public opinion needle a bit. Besides, I think it would be entertaining to watch the tea partiers twist in the wind trying to figure out which is more important: (a) making sure the hippies don't get their dope or (b) fighting the jackbooted tyranny of federal officers interfering with the sovereign Tenth Amendment right of states to police their own borders. Or something.