Originally Posted by jms698 
I just updated my Pentax K20d to the 1.01 firmware. However, now my 16-50mm f/2.8 lens no longer focuses. It makes a loud noise whirring noise and hunts to either end of the focus range. It sounds like it is trying to even force the lens to focus past the extremes of the focus range with the motor grinding when it reaches the end of the focus range. After hunting like this for a few seconds it gives up, failing to focus properly. This occurs in all lighting conditions. Previous to installing the new firmware there was no problem and the lens focused perfectly.
I wonder if there is a way to downgrade back to the 1.0 firmware?
In any case, I would suggest everyone hold off upgrading to the new firmware. It obviously hasn't been tested properly.
is it possible that you have a bad contact between camera and lens? If it is hunting like this it suggests that the data for focusing distance is not being fed back to the body as it should.
check and clean the furthest contact away from the lens locking pin, use a pink or gum erasor, not abrasive.