I’m seeing this, too. I just tried a fresh install of WPMU (2.8.4a) and tested Dashboard > Users > Add User (worked fine, email arrived intact).
After installing and enabling BuddyPress 1.1.1, doing the same thing to Add User results in a borked email:
Hi,
You've been invited to join 'WPMU Test Blogs' at
http://myserver.ca/wpmu as a subscriber.
If you do not want to join this blog please ignore
this email. This invitation will expire in a few days.
Please click the folowing link to activate your user account:
%s
The last %s isn’t being substituted by the activation link.