I would suggest you go look at:
http://www.sixapart.com/typekey/
It sounds like your idea is partially in play already, sans the widget thing.
I'm not sure I see any real value to this, particularly in today's "über-paranoid about my personal information being out there" world. I would bet that most don't even have the Autofill feature on their browsers turned on much less have a willingness to throw personal contact information out there in the form of a widget that would probably take 32 seconds to exploit in some way.
I really prefer to decide on a per site basis what info that I want to give (bank website vs. vw enthusiast forum, real email address vs. some gmail one that I don't mind being spammed). It sounds like I would have to edit your widget via another site each time, depending on the nature of whatever the registration is, just so I can send it, in which case, I'd prefer to have the information stored in my head so I can be more selective with what information goes into a form. I would also not like having to edit that information on another site.
Maybe I don't have any real vision and foresight as to the utility of this, though.