The first release candidate of the new WordPress MU 1.5 has just been released. The obvious major change is the new admin interface and password salting introduced in WordPress 2.5, but apart from them many bugs have been fixed.
There is also experimental support for CSS styles, something that has been missing from MU for quite some time due to XSS concerns. This function does the work of filtering out bad stuff and I would appreciate feedback, both positive and negative, especially with security concerns.
This release is quite stable, but there will probably be bugs still. Please only test it on a development server, and if you’re brave enough to put it live, make sure you have backed up everything first.