Infinity Ward has confirmed via their Twitter account that there will not be a Modern Warfare 2 demo before the game ships this November.
“No plan for a Demo before launch. The entire team is focused on polish all the way up to ship of the full game.”
Sounds like a good reason. If you recall, a demo was not released for Call of Duty 4: Modern Warfare and a demo for Call of Duty: World at War was released way after launch.
Modern Warfare 2 launches on November 10, 2009 for PlayStation 3, Xbox 360 and PC.