Go to the online lobby screen where you get the option to select 'Find Game' etc and then switch off the power to your router (don't just disconnect your ethernet cable). Press 'X' to exit the 'You have been disconnected... ' screen and then exit the game to go back to the PS3 menu. Plug your router back in and sign back into the PS Network. Then try COD4 again and it will work.
I honestly thought this was a load of crap when a friend told me, but it definitely worked for me and my problem was unable to play online when trying to find a game. so give it a go and see what happens.