To overshocked, I think they make dry ice the same way, since it is like N2, and is a gas at STP, (dry ice is just solid CO2 but you probably new that)
To Blade- like overschocked said, it isn't a good long term solution, because even though the mobo is out of the case, it doesn't matter. What you are trying NOT to short out is the mobo, so no matter where you move it, you've got problems. because the negative temps are directed on the CPU which is on the mobo.
The insulation that he stated keeps (for a short time like said) keeps the condensation from condensating as much and absorbs the watter that does condensate.
I've cooled the back of my GPU (just for fun, maybe you read my post), and while it works ok, it isn't worth it unless your trying to increase your GPU overclock.
As for the back mobo cooler, thermalright has something like this for their ix-14 i belevie. it cools directly behind the CPU, which makes sense, the only other place that would make sense to cool on the back mobo, would be the mosfets or the chipset. But you would get much better temps (and decent enough temps for almost all things) if you just replaced the stock cooler. Thermalright also makes these.
Any other place on the rear of the mobo just wouldn't make sense becasue it doesn't get that hot anywhere else, that i'm aware of.
(though cooling the backside of the mobo, anywhere else besides the cpu, is kinda crazy and would be hard to do.)
OK hope that clears some stuff up, sorry if i restated anything that anybody has already said.