Minor triumph. Hacker News dropped 2,095 hits on this post yesterday, which just shows you what a bit of well-directed whining can achieve; the fleeting attention of one million social-network Skinner-box pigeons. But yes. Anyway, Reggie makes a very good point in comments - why can't I subscribe to somebody's contact details and have them updated automatically? Amen! (He'll like that, according to his blog he's some sort of missionary.)
This shouldn't be difficult; you need only to specify a URI for updates as a field in the vCard, and have the client application check it (on start-up; every so often; whatever), or perhaps we could use XMPP, which would permit changes to be pushed out in real time. In fact, if the client was at all sensibly specified, if it found a URI without any contact information, it would fill in the fields from the data source it specified, so you could just hand round cards with www.example.com/firstnamelastname on.
Of course you might want to restrict subscriptions to your contacts, or provide both public and private versions, and certainly be able to revoke access to them; OAuth or similar is fine. I'm surprised nobody's done this yet. There are closed solutions, but it would be a pity to lock up all the data in a monopoly. In fact, perhaps the best way to deploy it would be to extend OpenID, associating a contact record with an identity URL and only divulging it with user permission. However, it would be nice to aggregate the information so that clients could register lists of contacts, and get a batch response ("No changes in your contacts" or a multi-vCard file of updates), especially as one of the affordances of such a system would be easy synchronisation between devices. In fact, it would obviate synchronisation as we currently know and hate it. (There's another desperately awful application.)
By the way, if you've just landed from HN, you might want to check out ORGANISE, my project for a Stafford Beer-inspired organising tool, and the specification v0.5, to say nothing of the Viktor Bout RSS feed and map.