In case anybody else ran into this (searched, couldn't find any posts) ----
After upgrading to WPMU 2.6 this morning, I was hit with 500 errors for the main site & wp-admin directories. Turned out to be a problem with the htaccess distribution.
Changed:
RewriteEngine On RewriteBase BASE/
to:
RewriteEngine On RewriteBase /
Hope this can help somebody out, thanks again all the others out there that have given me a hand.