I read Glyn Moody's writings with interest, but with his post Why Single Sign On Systems Are Bad I think he's made a rare slip. Here's a note to Glyn on why I think he's wrong on this occasion:

You conflate single sign-on (SSO) and single identity. OpenId lets you have SSO [and]{style="font-style: italic;"}lets you have a separate identity for each web site you visit. It's called Directed Identity, and Peter Williams gives a good explanation of it.

~~Yahoo! implement directed identity. Anyone with a Yahoo! account that signs in to websites using OpenId automatically has a different ID for each web site. When you sign in to the web site you just type yahoo.com in the OpenId box, and Yahoo! magically works it all out.~~

I got that last paragraph wrong. As Will Norris explains, Yahoo! implements identifier select and can give an opaque URL, Yahoo! does not support directed identity. Google does support directed identity.