Sumicom S350
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 6×10x1.65" 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.
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 fanless and uses the very low power consumption Pentium M. CappucinoPC carries it but not as a barebones thing.