Acrossair is another augmented reality application for the iPhone. If you don't know what augmented reality is yet then check out the video below demonstrating Acrossair.
The Acrossair Browser switches to a Google Maps view when you hold the iPhone flat. The map stays oriented to your current view as you move the phone around.
The usefulness of augmented reality applications for mobile phones depend on the quality of geo-data that they hold. Watching the video for the Acrossair browser it looks like they already have lots of data from a number of different sources.
Augmented Reality Roundup