MichaelMeissner:
If you are in a region of the world that uses GSM/GPRS cell phones (75-80% of the world uses GSM, in the USA AT&T and T-Mobile use GSM but Sprint and Verizon do not), you might want to use this shield, which would allow you to put in a SIM card: http://www.seeedstudio.com/wiki/GPRS_Shield. You will have to figure out how what cell phone options for the SIM card.
MichaelMeissner,
I'm trying to get this shield. Have you had some experience with it? I am curious what "unlocked SIM" means on their wiki. Thanks.