WordPress has been out in the 2.9 version for a few days now, and Donncha has just checked in the 2.9 code into the MU trunk. You can find it here: WordPressMU trunk
Scroll down to the bottom of that page and there is a zip link. Do not put this on a production site yet, but please do try and test it somewhere. Maybe a development site, or a test site on a spare domain, or locally on your computer.
The more people who test it now, the less bugs we have to deal with after releases.
There are some useful changes coming through, and some are especially useful for MU sites. Like the video embed. You’ll be able to ditch the plugins to embed video for popular sites. Please read the official release post on the WordPress blog for an overview and highlights.