I think the larger issue that is bugging you is the fact that user's data on the web is no longer owned/exchanged by the user but by the properties that the user interacts with. It frustrates me too, believe me. I would much rather visit a site that asks me for my FOAF (contains relationships to friends, profile data, &c...) URI, and a cert to authenticate me (this is called FOAF+SSL) - then automatically load and cache (for say, a month?) my FOAF profile data. Thereby never requiring me to enter anything more than once
except for my FOAF URI path.
Sad thing is, it isn't the providers that can't adopt it - the technology exists; it's the users. Users are locked into a model that is out-dated and are also locked into their own laziness and ignorance (many users would be scared off by the very mention of "FOAF" let alone maintaining their "own" FOAF file).
But, here are some of my counter arguments to your comment:
A) some users (I'm one of them) don't always use my actual IP to register on a site, I use anonymizing services often so I usually get frustrated by geolocating/ajax auto-fill forms and give up on a site that doesn't do any sort of graceful fallback
B) AJAX isn't effective when your user can't use Javascript (have you tried using your site with a screen reader? Can blind people use your site with ease?)
C) The act of typing in your address really is not that painful - I specifically set my forms with the autocomplete="off" attribute so user's personal data isn't being stored by their browser in the name of "convenience" (particularly billing forms that ask for your address data and cc data)