And if your ping is still high, it may be your Internet Speeds, or your on a Wifi connection, or you or someone else on the network is doing something to cause the connection to be slow. *Downloading/Uploading*
In CoD4, there are also several things you can do with the console to improve your ping such as increasing packets and reducing duplicated packets (as long as your connection is good enough in the first place)...
I would like to know as well. I play Counter Strike: Source and my ping is ok most of the time. But sometimes (I'm not downloading anything at these times) it goes from 40-50 to around 400-500.