It is probably one of these things where Americans swung one way and Brits the other. Another example is http://en.wikipedia.org/wiki/Aluminium#Etymology where Americans use the original British spelling and the Brits use the original American spelling!
But insure and ensure mean different things - they can't be interchanged!
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California. I even corrected Malcolm Gladwell once (which he thanked me for!)
I think I have found an explanation in http://articles.latimes.com/1998/jun/17/food/fo-60599 "and forget the fact that "insure" was nearly always spelled "ensure" in the 18th century."
It is probably one of these things where Americans swung one way and Brits the other. Another example is http://en.wikipedia.org/wiki/Aluminium#Etymology where Americans use the original British spelling and the Brits use the original American spelling!
But insure and ensure mean different things - they can't be interchanged!
Interestingly enough, as a California native, I got hung up on "insure" as well. I'm not sure I've ever used that spelling.
Not sure if it's something unique to my upbringing however.