The reason I am dissapointed in Obama's position on this is because I distinctly remember him saying that he would defer to the states on the issue of marijuana. But, he also said he wouldn't sign any legislation that would add a single cent to the national deficit. So, in other words, he's just full of shit like everyone else in politics. Say whatever will get the votes, then completely disregard the voters once in office.
The problem with Prop 19 is that federal law trumps state law. Period. If the state says it's legal, but the feds say it isn't, then it isn't. To get federal law overturned, you really need a Constitutional argument and in this issues, I'm not sure what that would be.
I think kill is on the right track. In this case, I think it would purely boil down to Fed vs. State, who has the right to legislate this particular issue.