BuddyPress.org

Are you using a brand new wp-config.php file–the one that comes with WPMU 2.7.1? Or, did you use an older version?

If so, there is an additional define line added to WMPU’s config file as of 2.7.1:

define(\'SITE_ID_CURRENT_SITE\', 1);

Also, before installing BuddyPress, but after upgrading WPMU to 2.7.1, did you got to Site Admin > Upgrade in site admin and finish the upgrade?