Perhaps it's just lazy (copy/paste) coding, as a Generation Zune commenter points out -- Xbox.com's source has the same line of code, with "xbox" instead of "zune" -- but why wouldn't Zune feature Gamercard compatibility? That Zune will be part of the Live Anywhere network has been rumored before, and there's no good reason to believe it won't.
We already know (see 2old2play) that MS Passport-associated Gamertag's will become your Zunetag. In fact, Microsoft has already reserved said Gamertags so that if you ever decide to go Zune, you're proper handle will be waiting. We're not convinced that Live Anywhere is a critical selling point for Zune, but tossing the feature in for Xbox Livies is a no-brainer.
[Via Gamertag Radio]