The Raspberry Pi board has plenty of GPIO lines available for hooking up sensors such as DS18S20 1-wire temperature sensors:

http://www.raspberrypi.org/forums/viewtopic.php?f=44&t=65137

I might also hook up a soil moisture sensor (measuring capacitance) in the future, even though we have more then enough weather data as it is.

The Pi camera has been a nifty addition to the system. We can now use it to monitor the plants and environment remotely!

http://www.raspberrypi.org/wp-content/uploads/2014/03/raspberry-pi-camera-module-300x225.jpg