I met a guy at Startup School that was planning to build a universal address book that would insure people and businesses knew where to send your bills, tax forms, and how to call you, etc. He intended it to be a tool for handling permanent moves, but I think there's the same set of problems. Making it worth anything requires huge uptake and getting huge uptake without value is impossible.

Personally, I like that every time I move, my junk mail drops off for many months (the "occupant" garbage still shows up, but it's easier to toss without thinking about it). I think that's the only thing I like about moving, actually. Having a system that allowed junkmailers to know my new address automatically would be a negative value, and I think the same is true of a site that allows constant contact with folks on vacation--I like being out of reach when on vacation.