Treyarch, developer of the Wii version of Call of Duty 3, has released the first images of the title in action on Nintendo's console. The images are scans from a magazine article, taken by a kind stranger, and as a result the quality is not the best possible but you can draw enough conclusions regarding the visual quality of the game on Wii.
Treyarch is fairly experienced at creating content for Nintendo's consoles and was behind the GameCube version of Call of Duty 2: Big Red One. It is expected that the Wii version of Call of Duty 3 on Wii will not be as graphically intense as the PS3 and XBox 360 versions although the developer has promised to work hard towards making it similar in quality.
These magazine scans will give you a hint at how that work is progressing but be warned that they may not be fully indicative of how the final version of the game will look.