I installed the nightly version of WPMU available above. It worked fine, but the error you saw above could be because “FollowSymlinks” isn’t on.
Add “Options FollowSymLinks” to the directory setting of your virtual host. If you’re running Apache 1.3, you can probably uncomment the “Options +followsymlinks” line in wpmu/.htaccess to do the same. I don’t think it’ll work in Apache 2 however.