A balloon3 with CUED IO board has been used as the central controller in a solar hot water system.
It provides monitoring of 12 temperature sensors over 1-wire and I²C and switches pumps on and off according to defined rules. All the data is logged and exported to a web-server for graphing.
This system can control pretty-much anything based on temperature readings and is in the process of being expanded to cover all heating control and power usage monitoring.
You can get more details at http://wookware.org/talks/hotwaterballoon-Fosdem2009.pdf