http://www.aplicom.com/gsm_modules.html
and a distributor, http://www.janus-rc.com/distribution.html. It looks like they sell a minimum of 20 units (from the aplicom FAQ).
For those that don't know, EDGE offers much higher data rates than GPRS (~300k), and falls back to GPRS if EDGE service isn't available.
I'd buy at least a few if SFE starts selling them and they're not $500 each
-Mark