I had a seat heater installed through mean city cycles, and have been very pleased with it. sometime in the past, I wrote up a post on it. the 2 stage heaters are the best IMHO, the low can be used to take the chill outta your buns, or boil the twins on high.
http://www.checkcorp.com/seat-heater...motorcycle.asp
http://www.meancitycycles.com/