Maybe it's not quite ASB that Paul could win, but it's close. It would have to take a major scandal in the Obama administration to give him a chance and that might only slightly improve his chances.
I'd be interested in seeing how Paul would debate Obama. I've seen the videos his supporters have posted on YouTube where they claim he absolutely demolished so and so and I just can't see it. Paul, to me, is a poor debater who gets extremely antsy when answering questions and his tone often grates - to the point where he comes across as whining.
I think, in a broader general election campaign, he'd find it difficult to articulate his more extreme views. He just doesn't have 'it' and while he would certainly gain support from a small minority, to me, he'd be to Obama as McGovern was to Nixon. His campaign would be rooted in the extremes and it would be easy to push back at the idea of him being president ... especially if some influential Republicans openly criticized his campaign. While I think most would get in line, I wouldn't be surprise if a few big-named Republicans openly said they could not, and would not, support Ron Paul.
So, guys like Newt Gingrich and even Mitt Romney might not endorse Obama, or say they're voting for Obama, but they'll actively say they can't, or won't, support Ron Paul.
In the end, if Paul ran against Obama, this is how I foresee the electoral college:
Obama: 56.6%, 419 electoral votes
Paul: 38.7%, 119 electoral votes
Other: 4.7%, 0 electoral votes