Go grab the new WordPress MU 1.2 from the download page now. This is primarily a bugfix update but it also has a security fix and all WPMU site owners are encouraged to upgrade.
I’ll post to the MU forums in a few days time with details of the security fix. It’s a short one-liner that can be applied within minutes if you can’t upgrade.
Update: After you update, go to “Site Admin->Themes” and click Save. That will update the allowed themes list because it was changed a few days ago.
Edit: a few issues with themes and .htaccess rewrite problem appeared overnight. These have been fixed now. If you updated to 1.2 then download the new 1.2.1 version and copy these files into your install: