Why would you want ANOTHER re-skinned CoD4? We want CHANGE.
Treyarch is using an improved CoD:WaW engine for Black Ops. They aren't touching MW2's horrid engine.
MW2 was a touched up version from WaW; its how its been since CoD:UO, one developer makes the engine, uses it, and the second improves and uses it. Treyarch is using IW 4.0, but did do some upgrades, in all reality the only major change was and is texture streaming.