I feel like Paul will either abstain or ultimately vote for Romney -- since he is an actual member of the Republican Party in spite of some un-Republican views, he clearly understands that the third parties are hopeless (now; there are paths they could take to try to get into a balance-of-power position, but they're stubbornly not taking them) and that the GOP is the lesser of two evils. He lives in Texas, where Romney will win anyway pretty much no matter what, so I think the first is more likely, or I guess maybe a symbolic vote for Johnson.
Paul could help Romney considerably in NV, CO and NH, though. So having not only his endorsement, but active support, could be advantageous to Romney. Especially if Johnson starts to surge in the polls, Romney might consider reaching some sort of deal with Paul on some of the issues we previous discussed a few months ago.