It really bothers me the way both John Stewart and Stephen Colbert say "an historic." Even if by some bizarre chance it turned out to be technically correct, it doesn't sound right and is harder to say than "a historic." If they weren't pronouncing the 'h' it would be one thing, but they are, and so they're wrong.
Unfortunately according to google, use of "an historic" is fairly prevalent with "a historic" at 7,780,000 hits and "an historic" at less than half that, but still a significant 3,440,000.
It's simple people! If it's a vowel sound, use 'an', if not, don't.