Ummmmm ..... No.
RIM won't let us play with any of the hard coded shortcuts, etc. If you are putting it into a case/holster that has the sleeper magnet, locking it is not really necessary. It doesn't really stop anyone from accessing it as the unlock instructions pop up on the screen. If you are doing so for security, you'd be better off to assign a password, but is many more keys to unlock then.
I personally don't bother as all my sensitive data is stored in DataVault, a password protected program.
__________________ Regards, Jag "Torch 9810 + Playbook 64gb"