Archiving in UIs
andrewacove.posterous.com
andrewacove.posterous.com
I suspect that adding an "Archive" button to the contact info view (to go along with the 'Share Contact' button) would probably suffice for the former.
For the latter, I'd probably use the 'Recents' page in the Phone app as reference. That view has a two-way button at the top to choose between "All" and "Missed" calls. Something similar for "Primary" and "All" contacts (though 'primary' isn't the best word choice) at the top of the contacts view might function well.
Interestingly, there already is support for one other view of the contacts list built into the phone: the Favorites list. The Favorites view forgoes the All Contacts view's A-Z# list, but ignoring that, Favorites is like the white-list version of Archiving's black-list. If every contact started as a Favorite when added, and "archiving" removed its favorite status, you'd have pretty similar results (though that's a pretty poor way to do it).
Those are my initial ideas for it. It would be interesting to see whether archiving could be sufficiently abstracted so as to be easily added to iOS's other default layouts (and whether there would be use in those cases).