I've always wanted to build one out of concrete. You know, 6 bags of quickcrete wouldn't cost all that much; I don't even think reinforcment bars would be necessary. It would absorb the heat well, acting as a gaint heatsink. It wouldn't exactly be a lan machine however.
That gives me an idea for cases. Case is all aluminum, and made for GOOD airflow without alot of loud fans. Make as much of the inside finned as possible. Attatch processor core to the case via a couple heatpipes. Yes processors would be harder to install, but I think it might take care of some of the thermal problems we're having.
Stock cooling(until I get a water block)
1GB PC3200 OCZ Gold EL(2-2-2-8-2 1T 5:6 Divider)
Sapphire X800GTO2(520/580 - Unlocked 16 Pipes)
Antec TruePower 550 vmodded
2x Western Digital WD-1200JS(RAID0)
Lite-ON 1633S DVDRW