.htm is a direct result of the old 8+3 character file naming restriction of MS DOS and some older operating systems. Search engines don't care whether you use .htm or .html, or even .php, so take your pick.
there is a small body of anecdotal evidence that suggests google prefers the .htm extension, but I've seen no real evidence of this. .txt files seem to do really well, heh, php all the way for me baby!
I imagine, so long as the spider can parse the text data it GETs, it can be indexed. I've seen some wacky extensions do very well.