There are times I wanted to configure my quadcopter using smartphone app, but I couldn’t install a bluetooth module because there isn’t a spare UART. Check out the Speedybee USB to Bluetooth adapter, an alternative to Bluetooth modules.
One great thing about Multiwii is, you can connect it to your computer, or Android phone/tablet via Bluetooth! It means you can adjust PID and other settings via the wireless Bluetooth connection, and there is no USB connection required. However the Bluetooth module might require some change of default settings (baud rate) beforehand.