If you're interested in creating Android applications to control your Arduino projects via USB, the Instructable by "raghvmahashabde" will be of interest. With the goal of controlling the PWM levels of an RGB LED connected to an Arduino with a USB host, they've demonstrated all the code requried and shown how to get it all together and actually working. Although the RGB LED may not sound like much, this example gives you the framework to build upon with your own Android<>Arduino interactions, for example:
If you're looking for an Arduino-compatible board with inbuilt USB host shield, look no further as we have the Freetronics USBDroid board:
Apart from being a fully Arduino Uno-compatible, it has onboard microSD socket and the full USB socket and host-mode controller. As well as the Bluetooth application described above, this is the ideal platform for developing peripherals or projects based around Android devices with ADK (Android Developer Kit) functionality, but without requiring a USB host controller shield stacked onto an Arduino. For more information and to order, visit the product page here.