We can't imagine why anyone would want to play a Game Boy game via their NES -- aside from the cool factor, that is. You can still find a Game Boy player relatively cheap, hook it up to your GameCube and get your handheld-on-TV fix that. But we digress.
Nintendo Age forum user bunnyboy is responsible for the Frankensteinian concoction of cool you see above: the NES RetroVision. It follows the same principles of the Super Game Boy, except in that it functions with the NES. By cramming the guts of a Game Boy into an NES cart, bunnyboy uses a custom board to translate the Game Boy graphics into a format understood by the NES. Utilizing a second controller, you can then change the border and colors displayed on-screen.
There isn't a page or any way to order one of these kits just yet, though bunnyboy promises that he'll eventually have some for sale. If you want one, you're going to have to fork over $130 plus shipping.