After moving a mailbox to another database you may start getting the following event log, even if the moving was sucessful:
Log Name: Application
Source: MSExchangeIS Mailbox Store
Date: 04/01/2011 21:10:35
Event ID: 10001
Task Category: Background Cleanup
Mailbox cleanup could not completely remove the mailbox for user ff783a8e-2dc9-4af7-bab5-435d4b841b2f (Root Fid 2-225C78C) following mailbox removal. Encountered error 0xfffffae8. Should this message continue to persist for the same mailbox, it may be indicative of a problem that requires further investigation.
To stop recieving this log just dismount and mount the previous and the current database for the specified mailbox.