Here’s a cool implementation of Android smartphone controlling an RC car via WiFi. I know there’s a bunch of ways to use Arduino with Android but this is an example that there’s no limits to what you can do with an Android device and your brain.
The RC car (battery powered) is completely controlled by Flyport. The module manages Wi-Fi link to an Android Smartphone and it drives the servo motors (PWM signal). On the following video you see the Android phone internal accelerometers drive directly the RC Car.