Today’s WordPress MU release is 2.8.4, a security release that fixes an annoying bug that allowed any user to reset the admin password. Your password was never at risk however so it’s more an annoyance than anything else.
Oh, thanks to everyone who tested the exploit on my blog. See? You didn’t get my password! 😛
Upgrade automatically from within your dashboard (first fix the upgrader if you haven’t updated to 2.8.3 yet), or download the new release from the download page and upgrade manually, overwriting your current install with the new files.
Edit: James Collins noticed that line 164 of wp-login.php wasn’t merged properly. If you downloaded 2.8.4, please grab 2.8.4a. Thanks James for the prompt feedback!