Schmap have just released their GeoTweeter application for the iPhone. This attaches a tiny-url style URL to your tweets, that includes your geographic coordinates.  Further information can be found at: http://www.schmap.com/geotweeter/

Note that the concept of adding location information to "Tweets" is not original. For example:

• http://geotweeter.com/
• Open Source: http://geotwitter.net/SignIn.aspx
  
• Display Mash up:  http://geotwitter.org/

I am not sure if any of these products will ever move beyond niche products. There are also potential privacy concerns, although like Facebook, these are probably solved by increased user awareness of the issues.