I just had this problem hit me this evening. First the machine seemed to freeze (one app after another not responding, till nothing worked). I rebooted, went back to work, and within minutes found the same thing happening. Oddly, this time I noticed Spotlight busy reindexing my whole internal drive. I shut down (put iMac to sleep with the button, then pulled the plug). I then booted to Snow Leopard install disk and used Disk Utility to do a repair (it found one file and one directory missing). After the repair, I booted back to the internal drive and logged into my normal account.
This is when I first got hit with the "Finder.app can't be opened. -10810" message. Since finder would not open I launched TextEdit and then Terminal from my dock. I then noticed that Spotlight was again trying to index the whole disk. I launched System Preferences from the dock and turned off Spotlight altogether. I fast-user-switched to my admin account and found Finder working fine in that account. I fast-user-switched back to my usual account, still no dice.
After reading a number of messages in this thread I began to wonder if an FSEvents problem might be a common thread (Spotlight and Time Machine both use FSEvents heavily). I decided that although killing Spotlight had not solved the problem, maybe now that it was turned off Finder would work if I logged off and back on.
I logged out of my usual account, then logged back in. Finder was working. Spotlight is still off (I'm afraid to turn it on, even though I really appreciate the searching). I'm not sure if Spotlight is the "cause," to me this feels like a deeper 10.6.1 problem. But I thought I'd share this experience in case it helps anyone else.