the thing I see here is not the idea of adding new hardware to the current pandora design (if I missed that, I stand corrected)... it's essentially tossing addons and software around until the pandora is "morphed" into a telephony device of some sort. I have racked my brain on ways this is feasable as well, because I see no reason carrying around a cell phone and spending $50+US (depending on provider) for unlimited everything on a cell phone which I either can't control the content of or can't currently hack to control when there is Linux, WiFi, VOIP and other technologies that could give me the same "unlimited everything feel" for anything from a few bucks cheaper to completely free (if I do my research well enough). Just can't justify a cell phone anymore after knowing what I am capable of with something like the Pandora and some nice Linux goodies. Being on the general topic, I'd hope someone would port twinklephone or ekiga (maybe even sflphone) to the Pandora for some nice VOIP action!
EDIT: Maybe more eyes can help see if the technology exists that I just thought about googling for: SDIO GSM or SDIO 3G cards... an initial search gave nothing, but I have to leave for work soon and wanted to spread the search of this out amongst the community to see if such things exist... if so, it would eliminate the need for any real hardware additions which would just leave driver/software support...