The big advantages of designing embedded products around PC technology is the simple development and quick time to market mostly due to reusing existing components such as: operating systems (Linux or Windows), drivers, development toolchains (GCC, .NET Framework, Java, etc) or even complete applications.
Encouraged by the success of Mini-ITX and Nano-ITX series, VIA announce recently pico-ITX. The new pico-ITX form-factor will reportedly measure 3.9 x 2.8 inches (10 x 7.2 cm) -- exactly half the surface area of Via's already small 4.7 x 4.7-inch (12 x 12cm) Nano-ITX standard, and considerably smaller than the original 6.7-inch square (17 x 17cm) mini-ITX standard.
If you are already using your PC only for a few dedicated tasks (VoIP, Media Center, etc) consider turning it into an appliance using one of the above motherboards. The tremendous benefits will include: low power consumption, no moving parts (most of these motherboards/processors can be used with a passive heat-sink and the HDD can be replaced with a flash memory), cool appearance … not to mention the personal satisfaction. A good place to start with such project is mini-itx.com