Have a look at the Friends list on your profile, acdc, and you'll see that the rectangular avatars aren't displaying as large as the square ones. Also, Andy is redesigning the portal pages of his sites, and there's going to be a "Meet the Members" section where a bunch of random avatars are displayed which can click through to the member profiles. You can see the layout on this page of one of his other sites:
Features : U2 Interference - U2 Fans, Pop Culture Webzine, & More
(scroll down to the bottom right). In order to fit in the rectangular block, the avatars need to be square.
The photo-of-the-month representatives will also be switching to the square avatars once we can get them uploaded to the galleries.
I'm still trying to get used to this square one of mine.