Here's another augmented reality iPhone app that might be slightly more useful for those of us outside of London than the Nearest Tube app we showed off the other day. TwittARound is a Twitter client (currently in beta) for the iPhone that uses your location info, compass, and the 3GS' video camera to place tweets close to your location in a realtime video view, so that the effect you get is like pop-up notes on the landscape. Very cool indeed. Unfortunately, it seems many of these AR (augmented reality) apps may never see the light of day, as some developers are saying Apple doesn't provide any public API calls for the live video (hence this petition from the burgeoning AR community).
There are other issues here too, though, even if Apple does open up all of the APIs needed for a project like this. Twitter doesn't actually include location information with each tweet, so what you're actually seeing (I'd imagine) is the location of each Twitterer. What you'd like to do with an app like this is walk around and use it to get information about what you see (look through the app at a line outside a concert hall, for instance, and see people talking about who's playing inside). But unfortunately, unless they have some way of seeing exactly where those tweets come from, they'll all still be from each Twitterer's location, not where the tweet was actually sent.
But maybe Twitter or some other app can start including that location (Brightkite is already location-based, and works with Twitter), and then we can get real-time information from where we actually are on an AR screen like this. There's still a few obstacles, but once those are evened out, the possibilities are very exciting.