Chuck Mortimore posts a movie of a login sequence in which an OpenID identity is represented by an Infocard in his Firefox Cardspace. When the user selects that card (after the Cardspace RP requests login) his extension uses the OpenID protocol to retrieve the necessary claims from the OpenID IDP, and then dynamically maps them into the Cardspace format for delivery down to the Cardspace RP.
Very nice PoC. It seems anything is possible when clients can translate between protocols.
Despite the data within originally coming from a 3rd party IDP, I guess the card that gets created will necessarily be self-issued - this because the step of translating from OpenID to Cardspace format would destroy any means by which the Cardspace RP would (reliably) determine origin.
The UI initially gave me the impression that the user actually presented their OpenID IDP credentials to the Identity Selector (and that caused great confusion for a bit) but its just how the OpenID HTML login page is embedded within.