like this one http://www.meshcomputers.com/updated/cubex_1.htm

obviously limited by the size of the casing, but size is important!

So Mesh have come up with their own name for a PC Shuttle then!
Do a google for "PC Shuttle" and you'll find alot more companies selling them (that's presuming you haven't done so already).

Can buy a barebones Shuttle for under £200 either AMD or Pentium, would need to add the DDR, CPU and GFX (if you don't want the onboard video) monitor and keyboard etc.

http://www.overclockers.co.uk/acatalog/Online_Catalogue_Shuttle_102.html

As for the shuttle itself, does everything you want it to really, can have decent CPU, GFX card, memory etc, just can't upgrade mobo, would need a whole new shuttle. Good for 12-18 months or more (depending on what you want out of it!)

Didnt pay much attention to it earlier when i saw cubex :D but sure looks like a shuttle to me.
Highly recommended, I'm on my second, sb51 and now sb61 with a 2.8mhz/800 cpu dual channel memory. Read the reviews on them and you will find they are not far off equivalently rated pcs in a full case. Ok they only have 1 agp and 1 pci slot but most things are available in usb 1 or 2, never really been lacking for me. Fit it with a fanless agp card ati radeon 9600 and a quiet running hard drive and you've got an almost silent pc. http://www.overclockers.co.uk/acatalog/Online_Catalogue_Shuttle_102.html for details