GyPSii has announced the launch of Tweetsii - an iPhone app that lets you see who's tweeting nearby, and also access information on what there is to see and do in any particular area.
Tweetsii hooks into other location-based networks like Foursquare and Gowalla, letting you access their information, but it allows the user to create their own locations and add comments and photos, rather than just "checking in". At festivals, conferences and events it could be quite useful viewing tweets nearby without necessarily following those people. Future versions of the app will also be able to check into the Foursquare and Gowalla networks.
Tweetsii is a free download for the iPhone, and it's available in the App Store now. There aren't versions for other platforms available yet, but versions for BlackBerry and Android are planned for the future. If you're desperate to code your own version for your favourite platform, you maybe be able to soon - combining the Foursquare, Gowalla, Twitter and soon-to-be-open Gypsii APIs.