@jayhule - I like the idea of tracing a shape- anything besides a generic swipe. If the main unlock screen part of Nokia's code is released/made open, it shouldn't take much to modify it to yield something more attractive. I also understand what you're describing with the locker combination.
On that idea I'll spin it and take it even further by making the "wheels" contain more than simply 0-9. Add the alphabet, allow character fonts (such as wingdings or from other languages) that require you to spin the correct combination of letters, numbers, characters, maybe even utilize FOSS clip art images.

Here's another one. Have small colored shapes- square, triangle, circle, etc. laid out so as to form a circle/ring with an empty target space in the middle. Now you have to drag the correct shapes into the middle, in the correct order. They can either be solid shapes or wireframe (go Asteroids!) and either vanish once moved to the middle or remain so you can see the color/shape combination being formed. Again, randomly change the colors of the shapes each time you have to unlock it so you have to remember the color sequence rather than the shape.

My visions are more akin to an active type of security interface, along the lines of a Flash app at least in design/function. I can't imagine Flash could be coded into the lock/sleep screen but it's fun to dream.

Heck, I'm somewhat doubtful the unlock portion of the device code will even be opened up. There are certain things that Nokia needs to maintain control and thus quality over. Wouldn't it be awful to install someone's custom security-based unlocker app only to find you couldn't unlock your N900 because of a bug?! Granted, perhaps these would be items to create then hand off to Nokia for approval so they can ensure it makes the grade. That's where sticking to basic customizations to the basic 'slide to unlock' design wouldn't pose any threat to the user experience if it failed- you'd simply use the hardware button instead.