In response to the Mail.app "breaking" I think it's a bug somewhere in the system prefs/ or mail.app.
Solution: you have to enter in all your accounts again (it works for me), and if you have a mac.com address, you need to erase the info in the internet prefpane, and enter it all in again (again, works for me, don't know about other people.
I've also had problems with the finder and system "losing" my passwords. For .Mac, every once and a while the finder asks me to reenter my password if i try to access .Mac by pressing shift + command + i. This is annoying; sometimes I have to restart before it works properly. Also, when getting email from the .Mac servers, it will ask for my password randomly. Meh...