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There's no real difference. In the old days of MS-DOS, you were limited to a three-letter extension, hence the need for .htm. Every web server in the world now supports both.
The only important thing is consistency - it is best to pick one and use just that (don't mix and match - it potentially confuses users).
[webmasterworld.com...]
No consensus was reached, but if there's an angle that didn't get covered in that thread, I don't know what it is! ;)