I am having the same problem, only I did not update my system and am still running 10.5.8. This problem seems to have developed on it's own, it wasn't after I installed an update or anything. I keep looking for an update to fix it, but no luck yet. I was hoping the security update this week would help but it has not. I was thinking of updating to 10.6 to see if that helped - but based on your experience, it won't.
Occasionally, it works, though. Not after I do anything in particular, just forget which computer I am on and try to start it up.
Repairing disk permissions (using the disk utility) hasn't helped.
I've been having this problem too. I thought it was due to Mobile Me "scheduled" maintenance last night, but now my online mail is working and my mail program is still crashing as it attempts to fetch my mail. I am using system 10.5.8 on this computer. My one at the office has snow leopard and was working normally this morning. I repaired permissions last night but no luck. Is anyone else experiencing this?
I may have fixed the problem ...
I set my att mail to just forward to my godaddy email account and set my incoming and outgoing to godaddy servers as well.
I deleted the at&t account from mail and just have a consolidated email account now.
I also manually deleted everything or put things into folders in both web email accounts.
It seems to have fixed the issue and I'm getting mail fine.
I'd recommend changing your password at your web mail client, then in mail.app and following the same steps.