Lock Mac after N mins of inactivity?

Use display sleep to save your screen; use the screen saver as a lock.

Let’s say you want to prevent burn-in after 3 minutes and lock the computer after 30 minutes.

1. Go to System Preferences > Energy Saver and set the display sleep time to 3 minutes.
2. Go to System Preferences > Desktop & Screen Saver and pick a lightweight screen saver (like Flurry). Set it to start after 30 minutes.
3. Go to System Preferences > Security and make sure “Require password to wake this computer from sleep or screen saver” is turned on.

Result:

• After 3 minutes your screen goes blank, preventing burn-in, but the computer is not locked.
• After 30 minutes the screen saver kicks in (although you won’t be able to see it because your screen is blanked) and your computer is locked.

You could set the computer to go to sleep after an hour and then make it ask for a password on wake-up.

This functionality was in the seed versions of Mac OS X 10.5 Leopard but was removed from the GM. One can hope that it will resurface in Snow Leopard due out next month.

It may have been just the interface that was removed from Leopard so there might be a hidden preference to do this.

As for third party utilities to do this, I haven't heard of any.

I checked the plist for the screensaver, but did not find any hidden settings.

On my mac, I do not have a password on the screensaver or on sleep, but I lock the computer when I know that I am going to leave it unattended for a bit.

To lock the screen on OSX, you need show the Keychain status in the menu bar. To do this, launch Keychain Access from the utilities folder. Open Preferences for Keychain Access, and select "Show status in menu bar" option. Then, whenever you want to lock your computer, just click on the menu, and select "Lock Screen".