Interesting, I thought he would run on the Libertarian Party's ticket, due to his base of support there and the fact that the Libertarian Party has better name recognition and fundraising capabilities.
Paul could conceivably score 5-10% on a national ballot running as a third party candidate. He already pulls about 30-35% when matched up against Dems as a possible GOP candidate. If he runs on a third-party ticket, he would be a serious blow to GOP support in the actual election as Paul draws most his support from the libertarian and constructionist wings of the GOP.
Don't think the Dems don't know this, either. In fact, the article referenced above cites the fact that Paul has received funding and/or support from 9/11 conspiracy theorists, anti-war supporters, and possibly from both George Soros and MoveOn.org. However, it should be noted that once MoveOn's Paul supporters were "found out" by other Paulites (which are made up of largely "small-l" libertarians who don't like Democrats), they were not greeted with open arms.
(h/t Spark It Up)