I found this nice pice of software here, but also I read some other,
similar implementations. ( typekey, lid, openID )
They plan to bring a new thing (currently) called YADIS, which is a
high-layer protokol, so a system will be able to "discover" the
available auth-methods in behind.
(e.g you have a FF plugin which supports you with the
single-sign-on/single-account thing.)
See http://yadis.org/wiki/Main_Page
greetings
.klemens