I was wondering the same thing myself. All chest freezers are manual defrost (according to the salesman at Sears!). I don't even want to think about defrosting a huge freezer full of food and cake, though.
I found this: http://home.howstuffworks.com/question144.htm
"A frost-free freezer has three basic parts:
A heating coil
A temperature sensor
Every six hours or so, the timer turns on the heating coil. The heating coil is wrapped among the freezer coils. The heater melts the ice off the coils. When all of the ice is gone, the temperature sensor senses the temperature rising above 32 degrees F (0 degrees C) and turns off the heater.
Heating the coils every six hours takes energy, and it also cycles the food in the freezer through temperature changes. Most large chest freezers therefore require manual defrosting instead -- the food lasts longer and the freezer uses less power."
Ugh. I guess a manual defrost freezer is the way to go, then.