While the addition of GSM capabilities would make this the best thing ever, from what I understand there is very little room inside for hardware mods. It could be theoretically possible, but honestly it'd be much easier just to tether it to a phone via bluetooth. You might want to take a look at openmoko/Neo Freerunner if you want a completely open phone. That project seems similar in many respects to the Pandora, and I know I'm getting one of those when they're out.
Also, I believe there are USB based WWAN devices that are data-oriented and may be usable with the Pandora. You *might* be able to make VoIP calls on them, but most providers tend to frown on that.