SUMICOM S350 Wow, they just shipped this, this is a Intel 865GV system that is the size of a standard CD-ROM. A whole PC in 6x10x1.65" if you can imagine that.
Only takes Celeron CPUs and requires a slim disk drive and 2.5" hard disk, but pretty amazing. It is $290 from "Cappuccino PC":http://www.cappuccinopc.com/slimpro-sp350.asp.
They actually run the Celeron at 66MHz and you can actually put a very hot Pentium in there as well which runs at 100MHz.
Personally, if money were no object, I'd get the fanless "Sumicom S625F":http://www.kingyoung.com.tw/ fanless and uses the very low power consumption Pentium M. CappucinoPC carries it but not as a barebones thing.