Eran makes a great point regarding multiple OpenID support as a necessity to avoid vendor lock-in. Sites accepting OpenID should find it in their best interest to allow users to associate multiple OpenIDs with their accounts. After all, as a site owner, you want to make it as convenient as possible for a user to use your site.

This is why I added multiple OpenID support to IPv6Links.Net. You can add and remove OpenIDs from your profile by clicking on profile from your links page after logging in. Each account must have at minimum one OpenID associated with it.