Now available in Cydia, for $0.99 from Bigboss.

Just downloaded it & it works a treat. It automatically enables the lock screen when you make or receive a call. You can easily get back to the normal menu's during the call simply by swiping in the same way you do when you are at your lock screen.

I recommend changing the settings to Automatic, as it really is no hassle unlocking if you need to enter more numbers or access something else.

So glad this has been released! I can now use my iPhone4 without accidentally muting every other call!!