I believe a "Lock Code" like S60 utilizes should be included. It is simple and effected and could use an on-screen keyboard for 10-key.

Something a little more exciting, would be something I have recently stumbled on for use in FireFox...FireGestures.

Sure, Unlocking the device with a gesture is cool, but why stop there?

Gesture 1: Unlock device.
Gesture 2: Unlocks and starts camera application
Gesture 3: Unlocks and starts A
Gesture 4: Unlocks and starts B
Gesture 5: Unlocks and starts C

I don't have an N900 yet, but I wouldn't mind seeing this type of application be system wide even and be able to contain a lot of user-generated gestures for complete customization and control of the device, assuming there is no technical reason 50+ gestures couldn't be used!