You have to hit it quite hard. In the lower left corner (the volumn button/lock switch side) but at the bottom corner of the player.
It took me 4 days to get my harddrive unlocked, each day i'd hit it harder and on a different corner, of course the it was the last day, last corner that actually fixed the issue.
I was really shocked with exacly how hard I needed to smack it. It would be nice if we'd get some new firmware, or maybe even a karma2. Things have been way to quite on the karma front.
mk1 30gb: 129 | mk2a 30gb: 040104126