When I log in to a relying party using my Yahoo account, Yahoo remembers the OpenID URL that I chose to sign in with. When I sign in to the same relying party again, Yahoo uses the same identifier again without allowing me to choose which identifier I want to use.
This means that if I have chosen ONCE to use an identifier that hides my email address from the relying party I cannot decide to reveal my email address to a relying party.
The problem about this is: I got a web site and I want my users to confirm their email addresses using OpenID. Users who have chosen once to use an pseudonymous URL cannot confirm their email address.