I've been looking around press sites and I've got a list of online-enabled games for Wii, both released and unreleased, all for Japan, UK and the USA. Online-enabled could mean extra features via WiiConnect24 or actual online multiplayer.
Released Wi-Fi compatible games (country available in brackets - eg. UK, USA, JP, AU):
Endless Ocean (JP)
Jissen Pachi-Slot Pachinko Hisshôhô! Hokuto no Ken Wii (JP only)
Mario Strikers Charged Football (UK, USA, AU)
MLB Power Pros (JP)
Momotarou Dentetsu 16 (JP)
Pokemon Battle Revoloution (JP, USA)
Unreleased Wi-Fi compatible games - subject to change - this may inlcude online leaderboards and extra content through WiiConnect24 or fully fledged online multiplayer.
Animal Crossing Wii
Battalion Wars 2
Dave Mirra BMX Challenge
Dragon Ball Z: Budokai Tenkaichi 3
FIFA 08
Geometry Wars: Galaxies
Ghost Squad
Guitar Hero III
Hooked! Real Motion Fishing
Madden NFL 08
Mario Kart Wii
Mario & Sonic at the Olympic Games
Medal of Honor: Heroes 2
MySims
NBA Live 08
Nights: Journey of Dreams
Nitro Bike
Octopuzzle
Rayman Raving Rabbids 2
Super Smash Bros. Brawl
Tony Hawk's Proving Ground
Trauma Center: New Blood