Just a day after WordPress 2.8.1 came out and here’s WordPress MU 2.8.1. The original WordPress announcement has plenty to say about this release, but what you need to know is this is a security update and a required upgrade.
This is the first MU 2.8.x release because of course there wasn’t a 2.8 one. Make sure you upgrade to stay up to date. The handy auto upgrade facility built in to the software should kick in but if not, go to the download page and grab the new zip file. Unzip over your current install and any database upgrades will take care of themselves when people login.
The WPMU Timeline is a good place to look to keep track of what has changed. Many bugs were squashed and features added.