OK, just been thinking about this problem for a bit and my best idea so far is to use a "double-decker" design. 2 hashing units, side-by-side, with the other two sitting on top. Allows for a cube type case, allows 4 intake fans (or perhaps just one big box-fan) and a very simple, straight-through airflow. Might not fit in a standard rack but it's incredibly more efficient and clean design.
Yeah, I'm thinking along these lines too. Just buy some off-the-shelf (boom boom!) shelves, power supply in the middle shelf, one unit on each of the other shelves with inlet fan fixed to the front. Cool as a breeze!
Could easily mod something like this to the right size for unit + heatsink.
I think I miss all the DIY gpu rigs
#modyourjupiter