Not much. Read inside.
I was really excited about seeing what Madden's GBA connection was all about. I hooked up the cable to my GBA and sat back as the game recognized that I had a GBA connected.
Well, in a nutshell it acts as a LCD scoreboard.
There are cheesy touchdown, first down, and even LCD field goal animations, and there's even a cheerleader dance part...but overall, it's a gimmick.
Regardless, if you have a GBA and a link cable, it's a neat little feature that the GameCube/GBA connection includes with Madden 2003.