Upgraded to Mac OS X 10.8.3 - Mail Upgrade Stuck

I wanted to wait for all the bugs to get worked out before I upgraded my Mac OS. I finally took the plunge.

Mac 10.8.4 upgrade went smoothly except for one problem. When I opened Mail, I got a dialog that it will upgrade my Mail database and that it will take a few minutes. That was 12 hours ago.

Mail is about 1/4 way complete, same as last night. The "Quit" and "Continue" buttons are greyed out.

What should I do?