A system that lest me maintain my contact details in one place, that other services can subscribe to for up-to-date info. Should also let me maintain who can see which details. When I change job, or lose my phone, or whatever, I can update my details, and my vCard in all my friends' address books gets updated accordingly.
Love this, and have been thinking about it for a while (after having moved literally 10 times in 10 years). I picture something similar to OAuth permissions, where at any point I can revoke access to all or specific pieces of information to anyone.
As an extension (and this is potentially where the money is), I can think of an intermediary service (a-la Google Voice call widgets) that allows entities to contact me without ever giving away my phone/address/etc.
Absolutely. Currently I have zero control over who has what contact details. I have good friends that still email me on an eight year old gmail address that I don't use. I need to be able to pull addresses, push new ones, have certain details only available to certain friends and maybe, like you say, have ways of letting people get in touch with me once without actually giving out my details.
This will happen once we have a federated social network with individuals maintaining their own little slice of the Internet. Please happen soon rather than later!