Using WPMU 2.7.1 with BuddyPress 1.0:
This theme displays avatars in different sizes. The avatars/gravatars of users registered on the WPMU system are displayed larger than readers who are commenting. The larger avatars obscure the view of comments: i.e.
see comments on this post
Typically, if I comment on my own blog but am not logged in, the avatar is displayed at the smaller, correct size. If I am already logged into the blog when commenting, the avatar is displayed at the larger, incorrect size.