According to a forum post over at the official Nintendo boards, the latest issue of Game Informer will have a piece on Call of Duty 3 where it is said that online multiplayer for the game will feature 24 player matches via Xbox Live on the 360 and PS3 online for the PS3. The Wii, however, will support the lowered number of participants by allowing only up to 16 players to battle over Wi-Fi. Now, without a scan or any other kind of corroboration with the initial post, we're forced to chalk this up to rumor at the moment and will update you as news further develops.
[Update: Go Nintendo received word that the information is in fact in the magazine, but is a misprint. Instead, the grim truth is that the game will feature no online multiplayer support on the Wii. Thanks Marc!]