If you want your PC to talk to your Android phone, use http://developer.android.com/guide/developing/tools/adb.html
. If you want your Android phone to talk to your Arduino over USB, you can use my MicroBridge library:http://code.google.com/p/microbridge/
It gives you bidirectional communication between your Android phone and an Arduino with a USB host shield. Shields can get bought either from SF for $25, or a slightly better designed and Mega compatible shield from Oleg directly:http://www.circuitsathome.com/products-page/arduino-shields/usb-host-shield-2-0-for-arduino/
N00b alert: this is a C library, and does not use the Wiring language or the Arduino IDE. If you want to use it, you'll have to learn C.