I don't have a cellular plan in the U.S. but I do have an unlocked smartphone which I have used a couple of times in France, with a SIM card bought in of the chain stores (forget the name). This allowed me to make and receive local phone calls as well as use GPS, and even check my email which is basically all I really needed.
Where would be the best place to get a SIM card--at the airport, at a kiosk in the city center, etc.?
Unfortunately, I am not very tech savy...
I don't know, for instance, if buying one ahead of time would work (and if it would arrive in time, besides).
http://www.greecetravel.com/phones/telestial/
I bought a 4Gite 3-in-1 SIM card from H20WirelessNow.com for 1 cents but really don't know how it works, either.