Gyroscope for the iPhone 3GS and iPad
Developer Vishal Shrivastav, a former Microsoft employee has developed the "Perspectiverse Engine". It is very similar to the Gyroscope feature unique to the Iphone 4. With the Perspectiverse engine, iPad and iPhone 3GS users can experience gyroscope too.
The software uses clever sensors that enable users to play games using their bodies as a sort of controller. He is even developong a game called Gyromaniac that uses the Gyroscope feature.
Here's a video to clear things up: