I invested some time learning Google Sketch Up, and am very pleased with the results. It takes a while to get some basic concepts down, but once you do, you'll find a very powerful and efficient tool.
Here's a screen shot of the latest of the designs of my fermentation chamber:
The basic idea is very similar to Jason's "Fermoire:" http://www.homebrewtalk.com/f51/fermoire-45567/
On the top shelf, I will place a compressor (probably an used one from a small ~2 cu. ft. fridge). The designs include a hole in the rear, which is not visible, to help ventilate the hot air created by the refrigerant system. Next to it I will keep a small tub of glycol (or other refrigerant-ish liquid). The evaporator coils will be submerged in this liquid. The liquid will be pumped to the other chambers, in the same manner as the Fermoire, where fans will blow the air across radiator fins.
On top of the middle shelf, a microcontroller (Arduino) will operate, reading temperatures from various probes. It will utilize this data to turn the fans, pumps, and compressor on and off, thus maintaining precise control over the temperature of the coolant and chambers.
The bottom shelf will be reserved for a bucket to use in conjunction with blow off hoses.
The blue things are recycled malt extract containers used as fermenters, measuring about 14.5" in diameter and about 26" tall. They hold about 15 gallons.
I will tweak the dimensions a bit, as this design is pretty large (3' 7.5" tall doesn't even include casters on the bottom!) and will also be very heavy. I hope to use a granite top, but may scrap that in favor of something less expensive and lighter.
I appreciate any thoughts or concerns. One thing I am particularly unsure about is how I will sample temperature of the beer. Ideally I read the fermenting wort itself, but that would be rather challenging. Sampling the temperature of the plastic, especially with a large fermentation, will certainly yield low temperatures.