To an extent, I think this is already possible - but only for inside a group of services. Take Google for example, and their iGoogle (is that what they call it now?). Smaller services don't have this luxury.
I'm actually going to be contented with a service that just provides central authentication to other services. It will provide all the basic information out there, even just username or password.
Instead of creating a new account in another service, say, Pownce, I just give it my ID (email should work best here) and it will automatically use it - central username and password.
I think Microsoft's Passport system tried to do it but haven't really picked up. I'm not sure why so it should be a good reading.