Are you using multiple accounts? If so then I may have the solution (perhaps single accounts have the same problem)
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 I just had to force quit over and over.
So after I returned my new MBPr 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.
Even if you don't have an iCloud account it's possible all accounts now need to be added through system preferences now instead of Mail preferences so it is worth giving it a try if you haven't already done so.
Of course what might have been easier is if Apple had got mail to come up with a message like 'this type of account should be added through system preferences' instead of crashing every time. It does make you wonder why mail even has a preferences with an option to add accounts?!