Upon upgrading to 10.2, I found that my new Mail 1.2 would not display any windows and was simply hanging until I force quit it. I cleared out my ~/Library/Mail folder and it worked again, rebuilding all my IMAP mailboxes automatically.
When I went to reinstall GPGMail.mailbundle I realized the real hangup -- that bundle causes Mail to get mad. No need to delete your mail, just delete that file from your ~/Library/Mail/Bundles folder, if you have installed it.
I also attempted to run my old Mail 1.1 app to continue to use my GPG utilities. No luck, it quit with no error message.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020830053915823