The rule is that you use "an" before a vowel sound and "a" before a consonant sound. "It is an honor." That H is silent. "He is a Hispanic." That H is pronounced.
I don't know any Americans who pronounce "historic" as "iss-TOHR-ic". (If so, do they also pronounce "historian" clipped? What about "hypothesis"?) And yet some insist on using "an historic" in writing, because, I don't know, it sounds more pretentious or something.
I'm a fan of language changing with the times. I could less care about split infinitives. And I start sentences with prepositions all the times. But language changes like that are a matter of style, while certain things -- like its/it's -- are just matters of being wrong. (Sometimes a subtle distinction, but I warned you this was a pet peeve.)
"An historic" is wrong.
(Further reading: http://www.wsu.edu/~brians/errors/anhistoric.html,