There is some dispute as to who owns the bug. Some feel the developers behind PHP should address it while others feel that the libxml developers should resolve it. I’m not really interested in getting into that argument. The low-down on the bug is that libxml 2.7.X has been rolled out into major...

Unlike the upgrade between regular Wordpress from 2.6 to 2.6.1 being non-urgent, this upgrade for WPMU *is* required if you’re on 2.6. One reason is it fixes a really nasty bug with deleting blogs. You can check the trac timeline to see exactly what was fixed & changed.  

Just before the last release, the check for unfiltered_html snuck back into the code base for MU. This means that if you’re using code since then, your users can insert malicious code into your site. Please read this forum post from Donncha for more details. You can update the wp-admin/includes/schema.php...

Seems the latest version is a wee bit broken in the cache area. It’s an older problem that was previously fixed, but soemhow slipped through again. It happens. Check the trac ticket here on the issue and the fix here.