Google does not seem to own circles.com. I wonder if they are planning on using google.com/circles or maybe another alternative domain.
Picasaweb.com is the same way.
I get your point, and it's not a bad one, generally speaking, but my gmail and picasaweb logins are automatic after I've logged in to any other google service -- I just cleared my entire browser history / passwords / cache etc., to make sure.
mail.google.com/
maps.google.com/