We did something similar to this recently. We setup an SMS gateway, which is a phone number that receives SMS messages and for each one, calls a php script running on a web server. It passes the original phone number and message to the script. Very simple to setup. The php script relayed the messages to a Pi that converted the messages into commands that it then executed on an Arduino Mega. We were letting visitors to our maker faire booth control the light and pump in our hydroponic exhibit by using their smartphones. Very cool.
To send messages back to the origin, you would need to use a similar gateway service, but I believe they are really inexpensive. Let me know if you need technical details.
tyler [at] hapihq.comhttp://hapihq.com