Zomb1337, COD5 sales on wii still slow, BUT STILL, just like Wiimusic had a "pathetic" beggining. As example, No more heroes 2 have less sales than COD5, but will have a second part. And treyarch/infinite ward are working on the same engine, thats mean less work.
And I must talk added this: Treyarch was working too in Spiderman Web of Shadows for many systems, and Quantum of Solace for multiple systems too, meanwhile Infinite Ward it´s just working on multiplatform COD6 maybe from 2007, this means they have the needed workhand to make the WII version, because they work exclusively on this game.