It's a great idea, I saw a dual twin core atom mobo w/1 slot that'd fit that or shoehorn into just about any Mac or ZFP case for $99 and a single dual core model for $50!

The HD20 case is a taller version of the APS ZFP HDD case I'm using for vertical expansion on my Classic/6500/projector hack and would be awesome coupled with any HobbitMacHack™!