Have you looked at a Rocket Mass heater like Melissa put in her new greenhouse, Ken?
I didn't understand how they work until I read the explanation on this page. (skip the video part, it's just an ad for his DVD's) Scroll down the page to where it has the diagrams and explanation of how they work. So clean, efficient and clever!
http://www.richsoil.com/rocket...
You should also look at using some kind of a solid heat sink to retain the solar and artificial heat that you're generating. Solid brick or concrete block walls on the north sides of greenhouses are very common up north, and both insulate better than a greenhouse wall, and absorb then retain heat that the sun generates during the day, releasing it at night.
As I've said before, my friend in Utah had great (economical) success keeping his greenhouse warm and humidified using storage tubs of water, and for the coldest weather he'd keep the water warm using a waterbed heater.
