Steve,
I am having problems similar to those reported by Niro...
I have about 5400 songs on my iPod, and regularly (successfully) sync the playcount data back to MC. However, when I add music to the library, and sync, my Playlists often disappear, and I am left with a single playlist entry of ' ', which itself contains a single ' '. The new music is all there on the iPod (using the Browse views). In addition, in MC, you can see the playlists from the iPod. Resyncing does not cure the problem.
I can manually 'fix' it by doing a 'Rebuild' of the iPod DB, and then re-syncing to get the playlists back..
I did some further testing, and have isolated the problem further. It only appears to happen when syncing AFTER importing about 10 or more songs into the library. It doesn't make any difference whether MC is shut down and restarted after the import. Deleting songs from the library and syncing does not cause the problem. During testing, I did not tick 'delete songs from iPod' or 'Sync Data to MC'. I can successfully add a few songs to the library and sync with no problems. Also, I am not doing any 'conversions' at all.
I have separately emailed you my system info and one of my corrupted iTunes DB files. I can simulate the problem at will if you want me to make any more tests.
Thanks as always for your excellent support,
Mark.