[jacquesm]

Ok, it's impossible. Now tell me how you're going to do it anyway and laugh all the way to the bank.

The fact that you can conceive of it means that it likely isn't impossible, merely very difficult and possibly non-obvious. But that's how pretty much every real success story starts. You really may be on to something here.

[mbrubeck]

This could be possible if web browsers (not just web sites) were aware of the standard and participated in the UI flow. Mozilla Labs prototyped something along these lines (not targeted for inclusion in Firefox 4, but possibly for the next release):

http://hacks.mozilla.org/2010/04/account-manager-coming-to-f...