Read that this is an issue with the controllers inability to 'pair-up' or 'synch' with the console.
Try this out:
1.Switch off the console by Pressing AND Holding the power button until you hear it beep then Press the "PS" button on the controller or,
2.Switch off the console and plug in the controller via the USB cable then Press the "PS" button to turn it back on again or,
3. Reset the controller by Pressing the 'Reset' button at the rear of the controller- it's the tiny hole to the left of the L2 button - use a straightened out paper-clip or something similar to do this.
Hope this sorts out your problem.
Call of Duty 4: Modern Warfare
16/10/2009 / Sydney, NSW