The openmoko project has a Contact App which is the addressbook. Check out that wiki page for an example of a good way to specify the look and the operation of an application.
What if the Contact App stored a foaf url as well? The contact app would then be a browser for a distributed address book. Would that be useful? If I were looking at the record for Joe Smith, I could tap and look at his friend's list. Sort of like I jumped into his own address book.
Oh maybe the best idea is that the foaf url can be used to refresh the details for that contact.
<foaf:Person rdf:about="#me" xmlns:foaf="http://xmlns.com/foaf/0.1/"> </foaf:Person>
There is an existing w3c contact schema for name/address/phone-number that can be mixed in with a foaf:Person.
i heard on irc something abount SyncML and the Contact App, too.