GameCube Hardware In Depth on Anandtech
plootid submitted linkage to a fairly technical look at the GameCube running over at AnandTech. You can learn about the hardware that makes the new Smash Bros. possible. Something about seeing Samus doing battle with Pikachu makes me want to know more about the PowerPC chip that powers the system ;)
Its still happening. They demoed this functionality recently with a new Kirby game, although I dont remember if the game is going to make it to the production state. Basically, you plug the GBA into a controller port for the gamecube, and if the game is GBA-aware, the developers can choose to do whatever they want with it. One VERY cool thing is that the Kirby demo included an empty 'gamepak' in the GBA that was en empty cartridge with a Tilt-Sensor in it. So you could use the GBA as a controller which could tell the Cube what angle it was being held at. Imagine the possibilities.
.. also consider that you could have one friend control the gun of a ship and use the GBA screen as a radar, and you control the flying of that ship. I don't know, there are lost of possible cool ways to use the GBA link, but we'll have to see if anyone stumbles across the aforementionned 'killer app'. Then again, who knows, maybe it'll just be Pokeman again. :)
But to answer your original question, the functionality is there; the only question is, can Game Cube developers find the 'killer app' for this configuration? I think the most useful implementation will be to share saved game data between GBA and Gamecube versions of the same game, but thats my limited imagination talking (and I dont know what typical savegame sizes are on GBA titles.) The football example you provided was good
Check cube.ign.com and gamespot.com, and do searches for "Game Boy Advance Link Game Cube" or something similar.
"Old man yells at systemd"
The 440GP was being considered, but Nintendo opted to go with the 750. You can read all about it here.