I know Paul has said this is off the table, but... is it really? Think about it. Paul's an idiosyncratic quasi-Republican who draws large chunks of his support from voters not inclined to back either of the major parties. His appeal extends far beyond the bounds of conventional red-versus-blue politicking into far regions of the electorate that rarely make their way into a ballot booth. Already, he's forced the GOP to reckon with some of his views by taking part in the primary. But if he were to enter the general, he'd likely also poach a number of Democratic voters and could well throw the election one way or the other. (Okay, he'd almost certainly throw it to Obama, if anyone. But still. The media is equally certain to spend many, many months trumpeting Paul's bipartisan appeal and doing the best it can to convince anyone listening that his candidacy poses a threat to both sides. Certain DC cliques are a sucker for this stuff and would respond accordingly.) Paul's well-known enough that he could make a reasonable case that he belongs on the debate stage with Romney and Obama, a la Ross Perot.
While, for most candidates, third party runs are pointless expressions of vanity -- money sinks that don't do much but risk sabotaging their ideological allies -- Paul's views diverge enough from an ordinary Republican's that running his own ticket makes a certain kind of sense. I don't really think Paul has any great love for or loyalty to the GOP, anyway. So why not maximize his impact and forge through all the way to November?