http://trac.mu.wordpress.org/browser/trunk now holds the just merged WordPress 2.5 and WordPress MU code. It's not complete and it's probably unstable, and it's certainly not suitable for the faint at heart.
Nevertheless, if you're willing to test it, you'll get:
1. All new WordPress 2.5 admin backend including media uploader.
2. Salted passwords.
3. Bug and security fixes.
4. Shiny new blog switcher created by Michael Adams
I've tested creating a new blog from wpmu-blogs, writing posts and watched that kses stripped out class, id and style attributes. Installation works of course, hopping from blog to blog does too.
If you're upgrading your test server you should open wp-config-sample.php and copy the SECRET_SALT and SECRET_KEY lines into your wp-config.php and set them to some random string.
I will need to check that those constants are defined but they may end up being site options to make it easier on people upgrading.
Let me repeat my previous warning. This is not to be used on a live server! Please only on test servers.