I have a similar issue. Everything is fine with my MBP as long as I don't run Mail.app.
For some reason I haven't figured out yet, when I do run Mail, everything locks up. Worse still, "Force Quit" doesn't help either. I have to force a shut down withthe power button.
This started after I upgraded to 10.8.2.
Are you using multiple accounts? If so then I may have the solution.
My accounts were set up within mail preferences on 10.8.1. When I upgraded to 10.8.2 Mail froze, nothing I did would unfreeze it. So after I return my new MacBook and got a new one the same thing happened again!
Here is what I did this time:
1) Create a new user profile
2) Put in your main iCloud log in
3) Do not add your other iCloud adresses using Mail instead go to System Preferences and Mail, Contacts, Calendars and add them there
Please note this will not work on your existing log in as when you go to System Preferences your old email accounts will cause it to crash as well as mail.
What might have been easier is if Apple had got mail to come up with a message like 'iCloud accounts should be added through system preferences' instead of crashing every time. Of course it does make you wonder why mail even has a preferences with an option to add accounts?!
Hope this helps.
I seem to be suffering from exactly the same problem. My cursor just locks up at random times when using Mail 6.2 (I am running OSx 10.8.2 Mountain Lion). It never happened before I updated the OSx. It is a real nuisance - the only solution I have found that works for me is to do a complete restart from cold. This is not at all acceptable or satisfactory. I am wondering if you, or for that matter anyone subsequently found a 100% fix for this?
I determined the actual cause of my issues. It wasn't really Mail-related at all in my case. Mail was just the the "unaffiliated app" that was most affected for some reason.
In my case, the underlying cause was a conflict between Time Machine and the new Box.net sync app.
I noticed that Mail was locking up mostly when Time Machine was running in the background. At that point, I started going over any other changes that I had made and I reemmbered that I had recently installed the Box.net app.
Once I shut the Box.net sync app down, everything went back to normal on my system.
Don't know if this will help anyone else, but I wanted to send my results along.
I eventually phoned Apple, and got some excellent advice from a very helpful guy there.
This "work around" seems to work fine for my MacBook, but whether it is a fix solution or for any other Mac, I don't know.
It would be best if Apple published some professional advice on dealing with this issue.
The advice I got was as follows:-
When you get the "lock up" follow the sequence.
Hold down together Alt, Cmd, R and just touch the power off (top right hand button) once.
Leave for a few minutes.
Then follow the sequence again, Alt, Cmd, R and touch the power button.
The problem has happened several times (I didn't keep score), since the original episode, but this solution has worked to clear the lock-up, and has got me using the machine again.
Which for me is ok, but there seems to still be an underlying problem which this solution does not solve.
Apple does need to address this and produce a fix 100%