Have you guys heard about this neat project: a sweedish modder named Anders Granlund recently created clone of Wolf3d running full speed on a GBC, contained inside an original gb cardridge. On the software side I think it's not even a port, but was inplemented from scratch and powered by a cortex M0 coprocessor with some additonnal RAM among other stuff built in the cardridge !
The man made a finished product with artwork, packaging, box and everything... quality work, would be pretty awesome to get a chance to know more from him directly on sudomod
Anyway here's a link to his build log: http://www.happydaze.se/wolf/