When I join a Google Group, Google asks me "What nickname do you want people in this group to see?". I pick something and I'm in. End of story.

Why can't I do this with every other site? I'd like to be able to go to foobar.com, click on Register and be taken to a Google page where Google says "What nickname do you want foobar.com to see?". The default is my gmail username but I can change it. I'm in. Foobar doesn't see my email address or a password or my address book contacts or anything else. Can people poke holes in this suggestion?

I think Joe the Plumber would love such a scheme.