New owners of the iPhone 3GS have yet another reason to make iPhone 3G users green with envy with the introduction of the app called Nearest Tube. The makers of the app (acrossair of the UK) call it an "augmented reality" tool for your iPhone, which is a pretty accurate description.
Using the iPhone 3GS, users living in London can turn on the video function, and instantly view live map indicators of where the nearest tube (or subway) station is. When held flat the screen displays the train lines underground, and when held vertically you are presented with actual signposts (Google streetview style) showing nearby train stations. You can see video of the augmented reality app in all its glory here.