I am sure this has been circulated over and over again in WordPress support circles, but I just came across it when I upgraded a test blog.
Basically, after upgrading to 1.2.1 if you try to login to your wp-admin interface, you might get a “Redirection limit Exceeded Error” which comes back with reboots and browser restarts. The error comes from the cookie that is setup on your browser from the old admin interface. Clear all cookies and you should be back to normal.
Uh, that doesn’t seem to do the trick.