Found an easy way to do this running 3.1.2 FW.

SSH or iFile into OS. Navigate to /System/Library/Carrier Bundles/Unknown.bundle/Carrier.plist.

Edit the key near top of .plist file - "ShowCallForwarding" to be <false/> and the key underneath - "ShowCallForwarded" to be <false/> - Reboot!

I'm ultrasn0w unlocked on T-Mobile UK so iPhone uses settings from Unknown.Bundle

If you are on a legitimate carrier then edit the .plist file as above in your appropriate "Carrier".bundle folder.

If you have a Custom Carrier bundle installed and you find the Carrier.plist file is already modified then edit the Unknown.bundle/Carrier.plist file as above as iPhone sometimes uses files from both the "Custom".bundle and Unknown.bundle folders (mine does).

Enjoy people!!