They aren't purely for decoration. If you look again you will see that the CPU is covered with a shiny bit of metal and that bit of metal is connected to the right heat sink by some funky new heat pipe...

...this wouldn't work with a P4 I don't think though, these EPIA processors really don't generate much heat (but then they are lots slower than the P4s).

I was wrong of course:

http://www.mini-itx.com/store/hush-atx.asp

Completely fanless 2.8Ghz P4 ATX machine from the same people...
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Remind me to change my signature to something more interesting someday