I think it is answered because the OP found the advice to purge the MessageUidsAlreadyDownloaded3 file, and relaunch Mail to work. They may have subsequently found things messed up again and not removed the Solved, but if so, that would be due to the issue with TM. Let's hope that TM will be fixed with the next update, but for those either using IMAP only accounts, or not using TM it is either not a problem or fixed by purging that aforementioned file. It should be noted, however, that some ISPs are known to find more problems with that file, even before Snow Leopard.
Apple would not mark any topic solved -- only the original poster can do that.
Ernie
Message was edited by: Ernie Stamper