Why not? Zune's primary distinguishing feature seems to the wireless connectivity designed to enable (DRM-constrained) song sharing. I wonder what control the user will have over such sharing, a binary on/off determined by enabling WiFi? Or something more granular?
Cardspace would allow a Zuner (can I trademark this?) to specify which identity aspects get advertised in which contexts. I might show a different set of songs in an after hours (I hear they can be open till midnight!) club than I would in an airport terminal. Other Zuners could define ACLs for their playlists and songs in terms of the required identity characteristics of potential recipients (e.g. if you advertise that you like 'Broadway Show Tunes' then you should just keep searching).
I'd be interested to hear Microsoft's story on how they see Cardspace working in a social world.