The iPhone has an always-on connection; that connection is to a GSM tower, which gives you exact distance from a fixed location right off, and often triangulation allows an exact postion. We're not so lucky; that solution won't do much good. But feel free to implement something simple like that, if you feel it's useful.

You'll need to install cron, install gpsd-clients*, add one line to crontab, and make about a 5-line shell script. Dunno about twitter, but you can FTP or SSH the data somewhere, if nothing else.

*<plug shame="none" href="http://www.internettablettalk.com/forums/showthread.php?t=20652">gpsd-clients</plug>

But this simple solution can't be developed into the sort of thing you're discussing. If you read between the lines, we've pretty clearly established an architecture of a simple daemon with plugins to accomplish the more interesting & complex tasks. That's how you make something that can actually grow usefully...