We brought you the story that an EA producer was not given access to Mii code for My Sims, and that according to him, no third parties were. The situation seems to have changed, because Sega Sammy is using the little guys and gals in its Jissen Pachinko/Pachislot Hisshouhou! Sammy's Collection Hokuto no Ken Wii. It's not the most exciting implementation of Miis, with no Fist of the North Star-style makeovers or actual in-game avatars-- in fact, it consists mainly of a Mii portrait tied to a character profile.
The game uses Nintendo Wi-fi as well, but for online rankings, which hardly counts as online play. We aren't ready to call that an implementation of third-party online play yet.