Yes, there is going to be Call Of Duty 6: Modern Warfare 2. It was announced like a month ago by Microsoft or the CoD makers, i forget It was anounced along with 2 other games.
There is debate whether it is going to be Future Warfare or Modern Warfare.
The release date is not specified but, around Fall of 2009.
I hope this cleared everything up, and if you want furthur proof just google "Call Of Duty 6: Modern Warfare 2".