Hey, quick warning.
Yesterday Steam reverted COD4 back to 1.6 (from 1.7).
If you have auto-updates on, or if you download COD4 from Steam today you will get version 1.6.
1.6 will not allow you to play on most servers, and can also cause other problems too.
See Steam change here.
This appears to be related to a pretty severe bug with many Call of Duty games allowing hackers to execute malicious code.
More details can be found in this Reddit thread.
According to Fraggy over on the CoD4X forums, this exploit is not even possible on CoD4. CoD4X thread here.
You can set your Steam preferences to not allow updates unless you launch the game (this will work assuming you don't launch through Steam). Tutorial here.
You will need the 1.7 executable file. Sheep posted one in the CoD4X thread which I assume is legit (not verified).