Just upgraded this WordPress blog to 1.5.1 which is based upon the 2.5.1 WordPress codebase. While WordPress MU 2.6 has been released, the upgrade process appeared to succeed; however, it would not allow me to login as any user except ‘admin’ to access the per-user blogs.
C’est la vie. WordPress 1.5.1 will have to do for now.
You might be better off doing xml dumps of your sites if that's possible with MU. Then you could do a fresh install, install the themes and such again, and import the XML dump. That's all assuming that works the same way in MU as it does in regular Wordpress. Granted, it's a lot of annoying work (recreating most everything from scratch), but at least you'd be preserving the content. I've gone this route with a few past wordpress upgrades, but there's a lot less to work on restoring back to normal with my single user blog. Also didn't hurt that I setup a staging instance that I could putz along in until I was ready to really migrate. Then again, is it really worth all the pain? :)