Re: Defualt member theme not working
Still thinking here, but a few more questions:
Are you using the new wp-config.php file that comes with WPMU 2.7.1? Or, did you use and older version?
If so, there is an additional define line added to WMPU’s config file:
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?