AcrossAir is an iPhone application that uses augmented reality to show you directions. In this video demo, they help someone navigate a subway station, but conceptually, it could work with all places that are identifiable to the device.
We haven’t looked at how the app actually works and don’t know how much storage is required, but we suspect that this might be taxing the battery life. the demo looks really cool, though. What do you think? (The application has not been approved by Apple, yet)