If you want to grab Release Candidate 2 of WordPress MU it can be downloaded directly from SVN using the following:
svn export http://svns.automattic.com/wordpress-mu/tags/1_0-rc2
or downloaded from the nightly builds page at http://blogs.linux.ie/download/wpmu/
If submitting bug reports, please be sure to mark them as milestone "WPMU 1.0"
Please read the following if you're having problems:
Upgrading: http://trac.mu.wordpress.org/wiki/UpgradingWpmu
Debugging: http://trac.mu.wordpress.org/wiki/DebuggingWpmu