It seems that MW2 has 4 hidden game modes that didn't make it in time for the games release. It's unknown if these game modes will be available later as DLCs for the Xbox 360 and PS3 users:
Arena - Round based capture the flag, with one flag in the middle of the map.
Global Thermonuclear War - There is a nuke in the middle of the map, and both teams have to get to it and set it off.
OneFlag - Just like capture the flag, but one team defends the flag and the other tries to capture it.
VIP - One team tries to get the VIP to the extraction zone, while the other team tries to kill the VIP.
Source: themodernware2.com