Well to answer kingwilly, if Paul is not the nominee I would not only vote for Obama, I would encourage everyone I know to vote for Obama.
The reason is simple: we are playing a two party game at this point. The third party idea is hamstrung by the rules the two party monopoly has created and what Paul has proved is if enough people get involved, you can actually start to move a political party in the right direction. The best way to make use of the momentum that Dr. Paul has created is to work hard at capturing the Republican party, or rescuing it if that is how you want to view it, from the establishment statists that run it now.
So if the nominee is Romney or Santorum, do I have a dog in the presidential race? Yes, and that dog would be Obama because we need to be looking to 2016 and if Romney or Santorum actually get elected president, then there will be no contest for the nomination in 2016. Having the Republican party lose the 2012 presidential election keeps the 2016 nominee position open for someone like Rand Paul to pick up where his dad left off.
So the short answer to why I would vote for Obama is because we need the Republican party open come 2016.