Depending on how much data you need to transfer between the two devices, I'd highly recommend a pair of ZigBee radios for low-bandwidth (<250k), super-low-power

Depending on how much data you need to transfer between the two devices, I'd highly recommend a pair of ZigBee radios for low-bandwidth (<250k), super-low-power applications. Specifically using a pair of XBee (series 1) radios in "transparent" mode, you get what looks like a serial device on either end. When used with a USB dongle, you can simply write your code to use basic ioctl ops on the USB device file, which works well across most POSIX platforms.