I have a WPMU installation (with buddypress)
I log into the my main WPMU install.
I then go to my sub blog and I have to log into that too!
I have to do this even when I log in as Admin to my WPMU install and the Admin is the same Admin that is set up as the admin of the sub blog.
I'm sure this is not normal and I have a feeling this has only started happening since using domain mapping for the sub blog in question.