Also, how about adding a external hard drive, RFID tag, IR.
I've been researching how to get IR on the gumstix. The pxa255 supports sir (slow infrared) on all of its UART ports. Basically it encodes/decodes all data sent/received to/from the UART into/from the IrDA format. There is also a "XMODE" which is the same as the sir mode but the pulse widths are only 1.6us. There are many inexpensive IrDA transceivers modules available that you could easily attach, but they are only designed for short distances.
The pxa255 also supports FIR (fast infrared) but it must use the STUART. Note: the STUART is not avalible on the GPSstix because it is used by the GPS module.
Another option is not to use the pxa255 encoder/decoders and modulate the signal instead. Most remote controls are modulated on a carrier frequency between 30khz-56khz. There are IR receiver modules that have built in filters at these frequencies. The pxa255 happens to output a 32.8khz clock signal on one of the pins, so I am planning on using this clock signal to modulate the IR transmit and a 33khz IR receiver module for the receiver.

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