Call of Duty 4: Modern Warfare was big. Huge. It also never saw the light of day on Wii, unfortunately.
Of course, Activision likes money, and it likes porting games to every platform imaginable, which is why this Amazon UK listing for Modern Warfare 2 Wii doesn't seem so crazy.
World at War sold great on Wii and apparently a lot of people play it, so it makes sense for a port of what will surely be an even bigger hit to the most popular console platform in the world this gen. It's just one more step towards Activision's plans for total domination.