I thought everyone knew about LAN through gameboy players by now?...
Anyway, there are two ways to play multiplayer through the GB Player.
1: you need 1 GCN, 1 GB Player, 1 TV, and players 2 - 4 need their own GCN/GBA link cables, GBA's, and I THINK their own cartridges...(GCN is more than powerful to power it all though...). And why not split screen 4 player...sometimes you gotta wonder what Nintendo's doing..
2: You need multiple gcn's each with GB players, multiple TV's...and that's it.
Go over to IGN Cube and search the mailbag. There's a fairly recent one asking about GBA LAN play. And the answer was gotten from Craig of IGN pocket, so i'm sure it's all true..