I have the same problem.
Rebooting does not work.
Throwing away the finder plist works - for a short while... not really sure how long, I get busy!
Going to Get Info, and changing the Open With option to Preview (instead of Photoshop) only changes the TYPE of generic jpg icon, from a psd designated one, to an even more generic one. Also, if I select "hide extension" - nothing happens, the extension still shows.
I have seen it suggested that it has something to do with Photoshop, but my other file types are okay. I still get thumbnails for pdfs, tifs and psd files. Not only that, some of the files that still have thumbnails are Photoshop jpgs.
It seems to be only the jpgs that are mixed up. Most of them are currently showing as generic icons again, but a few random images still show thumbnails. They are in the same folders as other files that turned into generic jpg icons... they WERE all thumbnails!
It seems to be the Finder. Throwing away the finder plist seems to be the only thing that affects it. I have heard to try Onyx and run the rebuild launchServices and cleanup scripts (remove caches) then restart. I will be trying this soon.
I also think it has something to do with the Finder because after throwing away the plist, the Finder will progressively begin to misbehave before I notice the generic icons returning. It's very subtle. The first thing I notice is that it will start to open windows in the same view as the last opened window - instead of opening it as it is set to open in View Options / always open in...
It could still have something to do with a conflict between Photoshop and/or Intel based macs.
In writing this and checking files, I have noticed that anything processed up until approx July 2005 maintains the thumbnails. Anything after that keeps losing it's thumbnail. This did not become an issue until I recently got an Intel mac using Leopard and Photoshop CS3.
Until then, I was using Panther and Photoshop CS (and later, CS1) on a PowerPC. It was the switch to Intel that brought it out, but it is happening to files that I had for 2-3 years before getting an Intel mac. The older files are okay, I didn't notice this at first.
So it seems to be the Intel mac combined with CS3 back to CS1?. It seems like we are caught between Apple and Adobe, but it seems to be more on Apple's side - as files that have never been touched by Adobe do not have thumbnails either.
I just hate to think that Apple has not addressed this! It used to be fine, never had a problem.
If Onyx works, I will return and let you know... (or if ANYTHING works!)