I'm British and I have never personally pronounced it "istoric", and to my knowledge I have never heard anyone pronounce it like that either. Is it a regional thing? I can only recall it being pronounced like that in US TV shows with comedy fake English accents.
"An hour" on the other hand makes more sense, as that is truly a soft H.
"An historic" is not only still acceptable in the USA, but it also (IMO) sounds better. The leading "H" is not a strong-sounding consonant, and is often dropped in both American and British English.
The 'h' is stressed in 'history', not in 'historic'. Try pronouncing 'historic' with a stress on the first syllable, it sounds wrong. Since it's softer in historic it's more natural (to me anyway) to use 'an'.
(Also, I'm french so I barely pronounce h's to begin with - so in my speech the stressed 'h' in history is kinda soft, and the unstressed h in historic is barely there.)
Which we do (in Britain and the rest of the English speaking world outside North America), so "an historic" it is.
e.g. from BBC:
http://www.bbc.co.uk/programmes/p055vr37