In 2008, the libertarians at Reason magazine were mostly supporting Barack Obama. I don't get how anyone could think that there is anything libertarian about Obama, but you can read their explanations. It appears to me that even political philosophers who pride themselves on "reason" can be swayed by mass hypnosis. At any rate, the Obama excitement seems to have worn off. They just announced their 2012 election choices, and they are not voting for Obama this time.
That does not mean that the libertarians are supporting Romney. The hard-core libertarians do not believe in voting.
Meanwhile, David Bernstein says that most libertarian law professors support Romney for the simple reason that only the Republican judges are reliable supporters of constitutionalism and liberty in controversial cases.