OSX lockdown after X minutes sleep?

If I set the (password locked) screensaver to come on after nine minutes, and sleep after ten, I can effectively 'lock' my iBook if it is left unused.

But I'd prefer something a little different - I'd like to have a password appear on wake, depending how long it has been asleep. For example, I close it, take it downstairs and open it - no password request. But then I close it and leave it in the lounge. Curious housemates investigate a few minutes later, but are stopped by a system password dialog.

My current workaround is to activate the screensaver on a corner when I knwo I'm going to leave the 'book lying around. But I don't always know when this is.

Any pointers to software that can do this? TIA.

-z-