Erica Sadun has done it again, creating a great app for the iPhone called "findme." What it does is constantly track your iPhone's location and sends it via SMS to Twitter. The idea is that if your iPhone is stolen or you want to enable your stalkers to keep track of you constantly, you can set this up and you'll automatically have a private Twitter page that shows the iPhone's approximate longitude and latitude.
It takes a bit of know-how to get it running (it's command line. If "curl" and "daemon" mean nothing do you, you might want to move along). If nothing else, it shows that I wasn't kidding when I said yesterday that developers will always find a way to do cool stuff with native apps.