Sega has announced 3 games that will take advantage of the GBA's multiplayer capabilities...
Sure the GBA is fun alone, but multiplayer is where you can have a total blast. Sega has realized this already with Chu Chu Rocket, and 3 of their upcoming games will have 4 player fun available. These games are Puyo Puyo Advance, Sonic the Hedgehog Advance, and Advance Columns. Sega...bring on that mad multiplayer fun!
There's no word yet if you will need multiple cartridges yet.