I don't think Paul would go Libertarian in 2012, but if he did, he probably would have had to drop out before the Iowa Caucuses to have any realistic chance of getting any serious votes.
Paul drains just enough votes from Romney in Texas to make the outcome there uncertain, but Romney would probably win it.
Obama easily has enough electoral votes to win otherwise, and even flips North Carolina from the IRL 2012 map thanks to Paul's presence on the ballot.
Obama/Biden 366 Electoral Votes 53%
Romney/Ryan 134 Electoral Votes 41%
Ron Paul/Gary Johnson 0 Electoral Votes 7%
Tossup 38 Electoral Votes