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":http://www.hushtechnologies.com/start.html. I bookmarked before, but these guys make nice looking mini-ITX.
"Logic Supply":http://www.logicsupply.com/default.php. A do-it-yourself resource for making and buying small and silent computer.