UPDATE 11 November, 2009: Apple states that the update to OS X 10.6.2 fixes the potential problem noted below.
When a user has been experiencing flaky behavior in a DEVONthink application I sometimes advise logging into a Guest account to see if the problem goes away in the absence of extensions that had been installed into the User account.
This is a warning note that the Apple forums are reporting instances where all the data in the regular User account may be deleted after logging into a Guest account that had been previously established.
Apparently, this problem is triggered only when a Guest account has been previously enabled. The solution recommended by some is to deactivate any existing Guest account before logging into a new one.
Still another reason to have backups of your computer data, such as Time Machine. :}