Webware’s Joshua Lowensohn pretty thoroughly covers and endorses the use of Twinkle. Twinkle is an application for the iPhone, currently you have to have “Jail Broken” your iPhone to install it, that is a location-aware Twitter client.
Not only does Twinkle allow you to follow your friends, it also allows you to “follow” those geographically close to you. I expect, come June, to see tons of location-aware applications for the iPhone.
Via: Josh’s Viddler video.