After upgrading to Tiger 10.4, I opened Mail and saw no messages. I could see all my accounts and folders but no messages and I could not check for new mail. Furthermore, quitting Mail would not work, though the application did not hang.
I decided that the third-party plugin HTTPMail Plug-In (for checking Hotmail accounts) might be the culprit. Sure enough, when I upgraded to the latest Tiger-safe version and force-restarted Mail, a window immediately came up telling me that Mail would convert my mailboxes to the new 2.0 format.
[robg adds: As a general rule, disable all plug-ins when doing a major upgrade such as this -- Safari, iPhoto, and Mail all have a number of plug-ins available, for instance. If you don't have a known Tiger-safe version, disable them prior to installing 10.4 via an Upgrade install.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050510121803822