If you're willing to run -current, check out the "Book PCs" at 
<http://www.hitech-usa.com/11_desktop_booksize.htm>. They are quite tiny 
and barely more expensive than similary-configured mini-towers. The 
downside is that there are no expansion slots, and the Ethernet chip in it 
still isn't supported by NetBSD. But, with the addition of a $40 
USB-to-Ethernet adapter (which needs -current, I believe; did this make it 
into 1.4.2?), it works fine.