WordPressMU uses symlinks to do the multi-blog thing. Blogs names are linked to in a “blogs” directory (avoids having the root directory writeable by the webserver!) which makes life easier on me writing mod_rewrite rules for various reasons.
Unfortunately, that requirement means that PHP has to be able to symlink directories which isn’t possible all the time due to security settings on servers. Therefore, here’s two htaccess.dist files that you can drop into your WPMU installation, then run your installation again (delete the existing .htaccess files)
Put this htacess.dist in the root directory: http://blogs.linux.ie/download/htaccess.dist
(If you’ve downloaded this file already, grab it again. I updated it so wp-login.php works, and wp-images show in the backend!)
Rename this and copy it to wp-inst/htaccess.dist: http://blogs.linux.ie/download/wp-inst-htaccess.dist
Delete the existing .htaccess and wp-inst/.htaccess and the install will run when you visit the root URL of your blog. You can delete the “blogs” directory too if you like! (Make a backup first though!)