Listed on the official Nintendo Wii site, out of nine first-party Nintendo games only two titles, Wii Sports and Mario Strikers Charged, are offering 4-player multiplay. The other seven games are limited to one or two players only.
In addition to the lack of 4-player games, the only games to offer online support at this point are Mario Strikers Charged and Battalion Wars II, which the final number of players is still TBA according to the official Wii site.
The lack of multiplayer support comes as a shock as the Nintendo Wii is capable of wireless internet connection straight out of the box. If the Wii is designed to be a family oriented gaming console, hopefully third-party developers will pick up the slack when offering more in the terms of online multiplayer support.