I had a thought to put the guts from a Mac Mini into a G4 Cube. I, however, did not have the money to do so, but a gent named Andrew did. Moreover, he did so with aplomb and a sense of style I don't think I could reproduce. In my quest to find the middle ground between Mini and Mac Pro, this is the closest I've seen yet.
With any luck, my own forthcoming attempt will also be a success. I am hoping to improve a bit on this design and add multiple hard drives and a second optical drive by stacking my components horizontally instead of vertically. Also, my unit will sport strategically placed EL wire and a more powerful power supply and distribution bus to allow for the addition of multiple drives. With any luck, and my electronic expertise, I should be able to envision my own Mini dream.