In the sequence of the post "GBC at Fana'briques LEGO event", Brian Davis commented about how he got so taken by the GBC idea, that he ended building a computer inspired on that.
According to his words, now he just need to make it compliant with the standards , to fit into a GBC scenario.
See below the video from this great exercise for a TECHNIC building,
This is a LEGO version of an old "marble logic" computer called DigiComp II (DC II). You can find more info (photos, plans,...) at Brian's Brickshelf folder.
The finished model weighs about 4,5Kgs (10 pounds) and uses 14 simple switches, 21 flip-flops, 8 half flip-flops, and a ball release mechanism. Later Brian built a slightly improved copy at the request of a museum, for public display of DCII.
You may find a more detailed and complementary info directly from DCII video description at YouTube.