The long awaited CoD4 demo is out 11th October 2007.
Have a look at the link below. I'm loving it already, looks so much fun.
Modern warfare, gorgeous graphics etc. There's a video download of the demo on there too, until it's released on the 11th
Clicky -> CoD4 Demo out Oct 11 9am PST
Looks like play are offering pre-orders for the full game out on the 9th Nov