I swear I saw a similar post but can't find it so sorry for repost...
I installed wp3 (godaddy) & activated (w/sub-folders) via plugin 'Enable Multi-Site'
Site is http://www.humantrinity.com/
I can login at main page so I added a theme & did some layout - seems all ok
Then I added first subsite - http://www.humantrinity.com/mainblog/
I cannot login @ ...mainblog/wp-login.php
and when going directly to the page the correct path (as above) displays but the page looks like default content w/no theme