Everytime I launch Mail it works for about 2 seconds and then beach balls. I do have Safari 4 Beta installed but I DO NOT have GrowlMail installed. Any other reasons on why Mail would be crashing.
Did you recently upgrade to 10.5? What email service is set up in the Mail application? Try setting up the email account from scratch in another computer user account. Additionally, here are a couple related articles on the Apple site: http://support.apple.com/kb/TS2486 and http://support.apple.com/kb/TS2396
If its IMAP, just pull the com.apple.Mail.plist in ~/Library/Preferences. And re-set up the account. If it was a POP account, I'd say to backup the Mail folder itself but IMAP accounts just re-index rather than re-downloading every single message.