I signed up with Club Cooee today, a 3D chat that doesn't have a normal window background, displaying it's 3D avatars, furniture, rooms, and it's 2D menu on the screen over top of whatever was running when you started Club Cooee. I don't like that effect at all. Why would I want to see a partially obscured word processor file or page of Google News behind a 3D "world"?
Club Cooee is unable to connect on my older lousier computer, but seems to work ok on my not quite as old better computer.
My avatar name there is ChuckBaggett.
Techcrunch had invitations to the Club Cooee beta at http://www.techcrunch.com/2008/12/04/club-cooee-releases-slick-3d-messenger-in-private-beta-grab-an-invite/ .