After I installed Jaguar, Mail would not display any windows or quit without a forced quit until (many painful troubleshooting hours later) I realized that the fonts in my ~/Library/Fonts folder were causing the problem. I'm still not certain if there was a particular font causing the problem, or whether I simply had too many (about 70). As soon as I removed most of them, my problems disappeared.
LaunchBar and MacReporter also stopped functioning until I removed those pesky fonts.
[Editor's note: I have a large number of fonts in my fonts folder, and haven't had any problems with Mail.app in Jaguar, so I'd guess this was a corrupted font problem. It's still a good trouble-shooting tip: if you're having strange problems in various apps, try emptying your local fonts folder and see if the problems vanish.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020904061902623