I’ve had my eye on this idea too (PhD student in computation biology). There are a few issues, most of which have been covered here with the same theoretical solutions as I found.
One extra problem though, on most motherboards the bottom two PCI-E slots are too close to plug directly into (since they are designed for 7-slot cases). One possible way around that would be to use a PCI-E riser ribbon card like this one: http://www.ably.com.tw/pdt/viewpdt.asp?abs…&cat=PCIE_RISER possibly with a straight riser on the card above too.
From my point-of-view, this certainly seems like a very feasible build, and as soon as I can persuade someone to give me enough money for it, I’d build one :)