The files changed in the latest update are as follows:

• tags/2.6.5/wp-signup.php

• tags/2.6.5/wp-login.php

• tags/2.6.5/wp-includes/post.php

• tags/2.6.5/wp-includes/version.php

• tags/2.6.5/wp-includes/wpmu-functions.php

• tags/2.6.5/wp-includes/feed.php

• tags/2.6.5/wp-includes/widgets.php

• tags/2.6.5/wp-includes/rss.php

• tags/2.6.5/xmlrpc.php

• tags/2.6.5/wp-settings.php

• tags/2.6.5/wp-admin/users.php

The latest version of WordPress MU can be downloaded here.