Same problem
I had this exact problem this morning, and I called tech support. Sure enough, they asked me to copy my mail files to the desktop and then had me delete that plist file you alluded to. My .mac account still existed when I started mail, but my other accounts did not and I had to create them again. Funny thing though, when I recreated the POP accounts, all of the emails were still there and I didn't even need the backup I made earlier.