Now, when I hang up from a phone call and the screen has gone dark, I either wait until the other party hangs up, or I change the orientation of the phone to get the screen to reappear, or I hit the home key and wait for the screen to come back and then hit the hang up icon.
Before the latest firmware update there was a problem with the phone taking a long time. Sometimes though if the phone call is long the screen will time out and lock. I just hit the power button to wake it up and then hit the hang up button. However if you have the latest firmware (2.3.340) if the phone call is short all you should have to do it take the phone away from your face.
You might also want to see what your sleep time is set to for the screen. If it is set to go to "sleep" in a short amount of time to save battery then you might want to lengthen it.
You can also set it so that the power button hangs up the call. Setting -> Accessibility -> "Power button ends call" <-- place check mark there. I haven't actually tried it since I use bluetooth 99% of the time and I actually hit the power button to turn of the screen while taking and then hang up with the bluetooth headset, but I am guessing this will solve your problem.