EPIA's finally come out with the Nano-ITX motherboard. A mere 12cm by 12cm; yet it's got 5.1 audio, VGA/RCA/HDTV video outs, USB, onboard LAN, and absolutely silent.
Of course, it's only got an 800 MHz C3 processor, so you couldn't make a cutting-edge powergaming machine out of it, and the CPU really doesn't have enough power to drive the HDTV-out port properly. But still: all that in less than five inches squared?
Still a bit pricey yet, but that'd make a heck of a nice toy.