Just stumbled across something interesting, Beagleboard.org..
It’s a VERY small motherboard that could be utilized for numerous things like building a small linux firewall (smoothwall or the like) or a media center (XBMC is working on something for this board). It’s all very interesting and definitely SMALL form factor, and at a price of $149 it is within a fair budget.
More info here;
http://beagleboard.org
http://www.youtube.com/watch?v=k-JUb6bb69E