*edit*
I think the culprit is the Letterbox.mailbundle for me. Mail seems to crash with it installed within 10 minutes, but again, it's just odd that it's doing this all of a sudden. I have used Letterbox with Leopard since the Leopard update was released almost a year ago.
I'm having the same problem. No issues at all in Leopard until... Safari 3.2(.1). So, I think that's the culprit. Though I can't imagine why it would be.