If the servers were in modular units such as semi trailers you could place however many you needed for the size of greenhouse range. Since greenhouses tend to be grouped in regions such as Cleveland, OH and Kalamazoo, MI you'd have your servers in a small enough area for a single person to be able to service them.
1. The depreciation cost of new hardware made this economically a no go. With second hand hardware, there was some number fiddling you could do to make it kind of work until it required any kind of human intervention whatsoever, then the labor costs dwarf the savings.
2. Computer enclosures are not designed to minimize the db/W/$. Low noise computers are generally achieved by reducing the power of parts and using expensive materials. High wattage computers tend to be extremely noisy. We couldn't be having a noisy heating system in our workplace, distracting everybody.
That sounds a like a reference, but I don't know it. Could you explain?
This is mildly like the situation where you should always slashdot-proof your web servers if it's easy to do, just in case some piece of content you have on there gets slashdotted. I feel like I say this so much that it's become my "stick of butter" story.
Also, could duct the heat into the clothes dryer maybe?
But given that water has a high specific heat, that could be a way to dispose of a lot of excess heat.
Does Anyone know their pricing? Qarnot Computing  does a similar thing and their solution is at €0.25/hour for “quad-core processors loaded with 16 Go of RAM”.
A house typically gets residential electrical service, ranging from 10-18¢/kWh in most of the country.
A server farm or datacenter typically gets general or industrial electrical service, at a savings of >5¢/kWh off the residential rate.
So, either you compensate the homeowner for the extra power cost (why?) or expect them to eat at least part of the cost.
This service would need some of the following to happen in order to be justifiable:
•A specific reason to have a server in a specific town (perhaps as a data collection or distribution center)
•Lack of suitable space and reliable power in an area, making residential or remote the only option
•A residential client capable of taking on general or industrial power rates (some apartment complexes or buildings might)
•Ridiculously low power costs and a constant need for low-grade heat (eastern Iceland perhaps?)
In the long-run, they're probably just banking on people paying a modest premium for "green computation."
Lastly, what about electricity? I mean these things have gotta use a whole lot more electricity than a typical heater. All of which, I'm guessing, is again paid by the homeowner.
If there's a developed home heating/computation market, homeowners would be happy to invest in a compuheater.
This brings me to your second point -- the problem is precisely that there ISN'T a developed home heating/computation market, which is why it would be difficult to convince people to use it (considering they have no idea if it will work and what trouble it could cause them).
I'm sitting next to an electric space heater that's heating our office. It's cold. If there's someone out there with a trivially installed image (think knoppix) that wants to pay for computer time, please send me an email. If the rate is sufficient to pay for the retail purchase of a computer and electricity in a winter season, I'm in.
I was considering doing this with my Litecoin miner. Its fans pull hot air from my GPUs at 85ºC which makes its room warmer than my bedroom. Maybe I should move it to my room and turn off the heater...
As long as the pricing pencils out, using computation to provide winter heat can be a win-win for everyone who needs computation, heat, and a planet.