Thanks for the feedback! A follow-up question:
On the FAQ page, I read this: “Also, if not using the RFM12B transceiver, digital pins D2 and D10 are free in all hardware versions.” Does the RFM12B transceiver come with the standard purchase of OpenSprinkler, or do I have to buy the pair listed on the website?
The idea would then be that if I have a wireless temperature sensor (such as this one: http://altelectronics.co.uk/shop/arduino/temperature-humidity-sensor-with-enclosure/prod_80.html), I would just have to modify the source code to receive this data.