First things first: Back when the Earth was young and recreation was watching the crust cool, the designation I recall was "1-RU" or even "One Rack Unit". After awhile we all got lazy and went to "1-unit", then even lazier, and started calling 'em "1u". Less to say, and less to write/type. A couple of thoughts come to mind. When designing a small satellite some time ago, heat dissipation was a considerable issue. In microgravity, one must accomplish cooling solely via conductive transfer and what comes close to black-body radiative dissipation. The satellite design was an external aluminum cube formed by layers which each held the electronics boards. Each board was multi-layer, but the first layer deposited was heavy-gage copper used as a combination common bus and cooling bus, to which all active devices were thermally coupled. Note that this took some careful work as some active devices had to be selected so that their heat-transfer element could be bonded to a common electrical connection. This thermal transfer bus was thermally connected to the spaceframe structure to transfer heat to the greater thermal mass of the spaceframe. Taking Doug's thought here a bit further, thermally bonding to the top plate of the case and becoming a little creative in its subsequent cooling (liquid or airflow) could result in better thermal management. However, this isn't a trivial design exercise (although it might be accomplished by the interested student). I would like to see a "standard" case that could accept some number (4? 8? 16?) motherboards and use large fans plus some form of liquid cooling to promote a thermal-neutral airflow. It could get interesting to achieve 1u density as things like power cabling would have to be engineered so one could make all the connections... I suspect this could be accomplished more readily than the pizza box design: I agree that the more you restrict flow by decreasing the space in a case, the more air you have to move across the components to accomplish adequate cooling. That said, Microsoft and Google had some very interesting results allowing clusters to reside at ambient temperatures, but I couldn't do that in Texas most of the time. »

Our experience with blades has been that while they're useful for HPC applications, they tend to concentrate heat discharge, making good thermal management a significant factor in planning and operations. Doing it over, I'd rather concentrate my systems in a multinode 1u chassis with more even rear-discharge air than a blade system with forced vertical exhaust. »