It happened. See Former National Review Publisher Endorses Obama. If you are an old-style conservative, you might find much to agree with.
UPDATE: Other endorsements of sorts can be found in this Bloomberg article, including those of three former SEC Chairmen (one Democrat and two Republicans). This quote is particularly striking: "Detached observers who watched [Obama] last week, especially in a Bloomberg Television interview, were taken by how conversant and comfortable he was on the subject, despite his thin record. Few detached observers came away with that impression watching the Arizona senator." Although, in fairness, it would be interesting to know the names and qualifications of the "detached observers" Hunt talks about in his article.
UPDATE TWO: Brace yourselves, for these must be the end times. George Will has voiced a quasi-endorsement of Obama (in the form of a rejection of McCain as, Will's words, the Queen Of Hearts.).