I didn't bother to mention the even bigger, better-known problem with OpenID: Users don't understand it. Only geeks understand it, and a subset of geeks at that.
Any plan which involves asking users to build a more complicated mental model of OpenID -- like asking them to understand the notion of multiple IDs aliased to the same account -- is just going to make that fundamental problem worse.
Any plan which involves asking users to build a more complicated mental model of OpenID -- like asking them to understand the notion of multiple IDs aliased to the same account -- is just going to make that fundamental problem worse.