I've actually created such a setup, I have 3 battery powered temperature sensors built on Arduino Pro Minis, they all communicate with a central RPi node via NRF24L01 transceivers, and the node then sends the different temperatures to a server via GSM. The sensors are pretty cheap to make, and if you put them in a sealed box they last for months without recharging the batteries (Currently 7 month battery life on 4 x AA batteries)

Here is some of the sensor code: https://github.com/cuonic/Arduino-NRF24L01-Thermometer