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.
http://hacks.mozilla.org/2010/04/account-manager-coming-to-f...
Nice implementation, poor sales/marketing.
Most websites simply can't see enough of a bang for an engineering buck they could be spending on something else (i.e. they don't even want to install it, never mind pay for it) and if it's done well consumers don't even see it so there's no money to be had from them either.
I'm sure we could have made it all slicker still but even Facebook login takes some justification and Clickpass didn't deliver anything like the value that that does.
With that said... isn't that really like OpenId?
I'll tell you what it should look like (the fact that it's impossible is not the point): whenever I land on a site that asks me to login, I get a menu of all my possible accounts, I pick one, and I'm in. End of the story.
Kind of like Dropbox being simple and intuitive when everyone else was building overly complex stuff.