I do not know about this. Please tell me about this.
Miyamoto showed it off at a few trade shows back in the day. It was a GC/GBA connectivity game that came with a tilt sensor that went in the GBA's cartridge slot; you'd tilt the GBA to control Kirby in 3D on the big screen, and at certain points Kirby would "fall" into the GBA and you'd play a little 2D minigame.
It was pretty much done, from the looks of things, but I guess they just decided not to release it due to the cost of the sensor, or because GC/GBA connectivity simply wasn't particularly popular.