How funny, I just started getting the EXACT same thing with my Intellimouse Optical and (of course!) came to the forums first and found your post right on the front page. Same kind of symptoms you describe: I left-click once, doesn't matter how fast, and the mouse thinks I did a really quick double-click. Files open with only one click, two copies of a program opens when I single-click their icon on the quickstart bar, etc. The most irritating thing is that it's totally sporadic for me--sometimes this will happen a few times in a row, or sometimes I can go for two minutes without a problem. It's not as troubling when I'm mucking around the desktop, but it's exceedingly annoying when I'm playing DiabloII and I allocate one extra stat or skill point than intended.
I'm thinking it might be a mechanical problem, which is kind of sad because the mouse is only around six months old. I thought perhaps it might need a little cleaning, so I blew into the button cracks, but that seemed to have no effect. The Microsoft website didn't seem to have any useful information in their mouse FAQ (no surprise there). Looks like it's back to my trusty old Logitech mouse!
/em bonks himself for "upgrading"