I also was wondering about how to keep water from freezing. I was looking on ebay and found this. I wonder if this would work well. It's water proof too.
http://cgi.ebay.com/Safe-Heated-Bed-Pad-Warmer-20W-Pet-Dog-Cat-30x40cm-/230534083793?pt=US_Pet_Beds&hash=item35ace6f4d1