I am planning to do the same by wiring an Arduino with Ethernet Shield into a Heatmiser control system.
I think the hardest part is getting the Arduino to talk to communicate through the Heatmiser Protocol. I am in the process of trying to achieve this at the moment: http://aglasspool.wordpress.com/2010/03/23/arduino-heatmiser-via-rs485/
If anyone is interested on collaborating on this, please do drop me a line.