I think Grant's dismissal of "an historic event" as "simply wrong" is misguided. First, let us dismiss Martha's objection that no one would say "an hippie"--of course no one would, and no one ever suggested it. "An" occurs in this optional case only with words beginning with H _on an unstressed syllable_, where, as Grant points out, the H sound is often elided in many dialects. A horrible drive led to an horrific accident. The N in this case serves to separate two vowels that would otherwise become an unclear glide or diphthong.
But even in dialects where the H is pronounced, the English H is a rather weak and breathy thing--not at all like the fricative consonant sounds of the Spanish J or German CH. In this case, "a historic" requires either a conscious strong exhalation or a weak glottal stop to separate the vowels, both of which feel awkward to my mouth. Otherwise, the words sound like "astoric".
So personally, I use the optional "an" on such words, because it sounds clearer and feels quite natural to me in speaking. And I see no reason not to write the actual words I say, so if I say that a hero performs an heroic act, I will also write it.
Right, Lee, but I was making it plain that it was wrong if you pronounced the H.
Also, the "a" is often pronounced as a nasalized schwa. The sound is somewhere between "a" and "an" but not clearly either. That could make the "a" vs. "an" choice more open.