"MTV Multiplayer reports that at a Massive (Microsoft-owned advertising firm) event yesterday in New York City, Activision Blizzard revealed a number of sequels to their popular franchises, such as Guitar Hero 5 and a successor to Call of Duty 4: Modern Warfare. The sequel will be named ... Call of Duty: Modern Warfare 2. And here we thought they were going to do away with the numbers! Unfortunately, no gameplay details were mentioned for either title."

PS3 Fanboy

Gameplay:

• FPS
• Builds off of CoD4 mechanics with slight improvements (it's CoD4 with a new coat of paint)
  
• Introduces a few new weapons and mechanics (cover perhaps?)
• New multiplayer modes
• Has something to do with the Middle East

There.  I know nothing about the game and I bet those are the gameplay details.

And maybe they're redoing the numbers to make it seem like Activision isn't whoring out the franchise so badly.

Considering how much COD4 sold I'd have thought more people would be interested in COD4 : 2.