if your mobile phone platform and carrier supports JAVA, you shouldn't have much trouble whipping up a simple app to make use of the bluetooth SPP profile, which essentially replaces an rs232 cable with bluetooth.
The hardware is expensive, I recently paid roughly $50 USD for a Sena bluetooth module. It is a nice module and is worth the price, the range is excellent.
USB slave functionality is easy to do with microcontrollers, either using an external USB <> rs232 converter chip, or using a uC with the usb hardware built in (PIC 18F4550 for example).