I did actually do the math, and assuming a 100-watt processor, the result I get is that the air (at 10 CFM) coming out of the processor's cooling module will be 19.2 (!) degrees hotter than the air coming into the case. Up the airflow to a painfully loud 20 CFM and you still get about 10 degrees of temperature rise.
MikeC is right - with BTX, intel did what they needed to so that their processors could actually function - at the expense of practical cooling of the rest of the system.
Actually it is even worse than that: Intel guidelines call for 115W cpu + 38W vreg. So the intake air will absorb 150W!!!! As I was reading the pdf, I thought NOOOOOOO!!! I did the calculation as well, for 35cfm air (Intel guideline) temp rise is 7.5C. Now this isnt tragic, I was really surprised. It is reasonable. Well lets say borderline reasonable. BUT:
A 90mm fan cannot give 35cfm quietly. And according to the BTX form factor, it will be closest to you and drawing air through a non-restrictive bezel. BTX may be great for Dell and HP and others, but for SPCR, I think it might be quite a headache.
Even if we were able to use a 120mm fan instead of 90, the nexus 120 gives 37cfm at 12volts. (I've never heard the nexus.) The question is: Is the Nexus 120 mm at 12 volts quiet enough for you?
The good: Linear airflow, low no of fans, smaller case.
The bad: Cooling design seems inflexible(Bad news for SPCR)
The ugly: Cpu heat exhausted into the case.
Notice that the BTX seems to be space optimised for a 1CDROM, 1-2HD desktop case more than anything else. Cooling the cpu is highest on the priority list and this indicates that prescott is here to stay...
I think a quieter/cooler design would be something like this: (Yes, I know about the Lian-Li v1000)
The idea is to have the cpu socket really close to the back end of the mobo, use a 120 as CPU/GPU (think NCU-2000+vm-101) cooler/exhaust fan and get the IO shield out of the way. Airflow would be much closer to the mobo, and you could even have under mobo airflow as Intel suggests in BTX. I obviously do not know the complications (if any) of placing the IO shield like that. But sice mobos sometimes include pci card type of expansion cards, I dare say its doable. The PSU would have a fan but it would be at the inner side of the psu. There would be two seperators running the whole lenght of the case to keep airflow tidy, one between PSU/IO shield, other just above vidcard (or maybe 2nd pci slot.) This way, psu fan and 120 fan would be ducted for linear airflow. Have a large filter at the front to pull air in and volia! You could even reduce the case width to the height of the pci slots by using a shorter psu ~12cm. (Just draw a line straight down the left side of the pci slots and 120 fan)