I'm about to check in 250 changed files merged from WordPress trunk. This is what will eventually become WordPress 2.6, but it's quite stable already.
However, if you're running MU trunk on your production server please do not blindly update now.
The final pre 2.6 changeset is [1327] so if you want to use that, update like this.
svn update -r 1327
Testers of the new code are more than welcome to bash on it on dev servers but please don't try it on a live site!