Sorry, don't mean to show off... ok maybe a little 
Yeah your rigs are the inspiration for the ideas I got in my brain. I was thinking of having the twist of mounting MB "standing up" so the GPU "exhaust" (I use 5970s exclusively) points "up". Best way to describe it would be blade server meets mining rigs.
Cold air would flow under the bottom of MB, be pulled into GPU intake, warmed up, and blown out the top where it would have nowhere to go except the exhaut out. I wanted to makes some changes to my garage anyways and now my wife has seen other people have "organized" rigs it might happen soon rather than later.
People say that hot air rises, which is true to an extent, but you can manipulate it with fans and ducting to go where ever you want.
PS: Invest in a big box of smoke pellets like these:
They are great for working out what your airflow is doing.