Like many others, I've just installed Leopard on my computer and I've been very eager to try out all the nifty new features. I think I've discovered a bug as well as a work around -- so if the following happens to you, at least you will have a recovery mechanism until Apple fixes it.
I created a To Do item in iCal, then launched Mail, only to have it crash after about 20 seconds. I examined the crash report logs, and found an exception where Mail was attempting to assign a To Do to a non-To Do-enabled mailbox.
To fix the problem, I deleted the To Do item from my calendar and restarted Mail. In Mail's Preferences on the composing tab, there's a selector box that enables you to assign your To Dos to a particular email account. Mine was set to "Last Viewed Mailbox." I changed that to my dotMac account, and Mail has been stable since.
[robg adds: I can't confirm this one here as my mail is one thing that hasn't yet made the big migration.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071027063232757