I suffer with the same problem and I did hope very much for the fix with 10.8.3. But no, it's a shame for Apple.
I have part of a solution discribed here: https://discussions.apple.com/message/20604110#20604110
Maybe it gives new ideas to you? Please let me know when you know how to resolve the problem without loosing information in the libraries.
As you're saying in the other post, this is definitely not a stable solution. Once you touch something that iTunes doesn't like everything goes broken again.
At each update (iTunes or OS X) it is the same story, stuff gets nonsense.
For me I'll get an iMac in a couple of weeks (ordered already, 6 weeks delivery, welcome to China) and I'm having iTunes Match so I'll start with a scratch iTunes Library and will download the tracks (with ratings and comments) so it'll be alright. But a final solution might help other people.
For me, the solution that worked:
1. Get back a 'iTunes Library.itl' file from before the update with Time Machine. This restored the thumbnail from when we click on a song in Finder and the iTunes Screen Saver, but the files themselves still don't have any thumbnail.
2. Thanks to iTunes Match, delete all the music from the library (it doesn't remove any rating, comment or playlist) and download all the files again.
Now everything is back there, all thumbnails in any case. Probably until the next update of ITunes or OS X.
Well, as it takes ages to download all the files, I wanted to try an app I've found on a French website: http://www.titanium.free.fr
The app is called Maintenance. I stopped the downloads, ran the app (took about 2/3 minutes), restarted, wow, everything is there back to normal. Not sure what options made it work, but it works. Excellent app. But shame on Apple for screwing this up for such a long time and doing nothing about it.