New in the NYCR vending machines are Teensy 2.0 boards. They have ATMega32U4 chips, which have the built in USB drivers and, via LUFA, can appear as any USB device, not just a serial communications device. Want to make a MIDI device show up as a USB keyboard? Or a core memory as a mass storage device? You can do that! The USB doesn’t consume a UART, so there is still a serial port available for interfacing with GPS or other external RS232 devices.
PJRC makes the Teensyduino plugin for the Arduino IDE and a set of compatible libraries so that you can use it with your Arduino sketches. Or you can drop into straight C and take full advantage of all of the AVR pins.
Update: They are very popular! Three were bought during Craftnight tonight.