Ok... I totally confused by davidgro's problem and fix. When I press the power (while in KDE) button and select 'LOCK' then I get a pop-up window asking me for a code - no matter what I enter, it says incorrect code and refuses to lock the device. When I select 'LOCK TOUCH SCREEN AND KEYS' everything behaves normally as if I had just used the slidy switch (the one to the far right of the power button) on top of the device. It also unlocks appropriately using the slidy switch.