We're not quite there yet, but soon you'll be able to use any reasonable user identifier as an OpenID. Most importantly, email addresses. Dirk just wrote a great blog post explaining "Email Addresses as OpenIDs" which goes into the nitty gritty. Basically, this all just needs a finalized XRD spec to rely on, and adoption of same (and ability to use acct: URIs) in the next rev of the OpenID specification. The upcoming IIW will be an great opportunity to make some progress on these.