Johnson would have a great chance at getting the Libertarian nomination, and he does seem to be considering it. They seem to like him, and if they gave it to Barr, they'd have no trouble giving it to Johnson,a politician who they have always agreed with with on most issues.
I don't think he'd do much better then the average Lib though. Paul would, but that's a large part a personal following. Plus Paul would get some media coverage, being a pretty strong Republican candidate who switched parties. Johnson, not so much.
Now if Mitt's nominated, I could see the Constitution Party doing really well. but I don't think Johnson would be the golden bullet for the Libertarians.
Well, I'm sure not voting for Mitt or Cain... be it primary or general.