I don't think a third party candidate will be viable this time around. But depending on the outcome of the Republican Primary, the average voter might be more willing to look that direction in the near future. If Romney wins (which I still doubt at this point) a push for a more conservative and libertarian influence will again be pushed forth from those of us that are not happy with 'establishment' candidates.
In order to get the average voter to go away from the "establishment" you would need someone with the charisma of Herman Cain, Mike Huckabee, or the "new" Newt Gingrich. Johnson isn't that person, but someone with similar beliefs (more libertarian) on many of issues might be. Ron Paul isn't that person either and Ron Paul isn't leaving the Republican field. He is hoping for a prime time speaker spot at the convention to help his ideas get a bigger audience. That can only happen if he wins at least a certain number of states (or delegates). He has more influence by staying in the Republican Party.