Mini ITX
These systems are the closest that you can get with a PC form factor to an iPod mini. Some resources include:
mini-itx.com - news. A great site showing both mini-ITX and the upcoming nano-ITX from VIA.
Hush Technologies. I bookmarked before, but these guys make nice looking mini-ITX.
Logic Supply. A do-it-yourself resource for making and buying small and silent computer.