I know MK:DD, Kirby's Air Ride, Crash Nitro Kart, and Custom Robo will all feature LAN play. Supposedly Nintendo is going to try to focus a lot more on LAN play in 2004. Also the Big N has an agreement with Gamespy for online middleware, though the details are a bit vague. Tunneling software is also now in the works via resourceful users (that probably want to play MK:DD online).
Personally, I like LAN play since it's free. Online play is cool, too, but I'd rather not have to pay for it. I'm already paying 40 bucks a month for broadband access, so having to pay money to play on top of that (when almost all online PC games are free, save for MMORPGS) is not enticing in the least.