I have been in many Identity Management projects over the years. Many times there is an option for a user to use an id from an existing system, or a new id. I have realized that the id and the domain attached to it just don't matter.
Let me be clear when I say that it doesn't matter, it does matter that the id is unique to the system. It doesn't matter what domain the user has after the "@" symbol.
The important part is the end user experience. Let the end user choose their username. Maybe they have one they like to use. I like OpenID and everything that I think it will deliver over the years. I think the ease of use and decoupling the username from the system is what I have wanted for years.
The decoupling is possible without OpenId.
For example, I am using a yahoo user id to login to manage my google blog. This is not OpenID but it sure did make my life a lot easier. What does it matter to Google?
What does it matter to yahoo? Nothing. Its just an ID. I am so happy Google didn't force me to sign up for the gmail account in order to use Blogger. That littel bit makes a difference to me.