I second this option, but I think a sound wouldn't be a good idea for 2 reasons 1. If the tablet is muted, you won't hear anything. 2. The sounds may be too loud if your someplace where you should be quiet (meeting, theatre etc)
Instead of a sound, I was thinking of the LED. After watching the flipclock demo video, you can see the different colours the LED is capable of. Something like 3 yellow flashes of yellow when the the sceen is locked.