If nobody else has one yet this would probably be a pretty easy app to build; not having an N900 I couldn't test it, but in theory it's easy. The bigger question would be about practicality:
1) Doesn't the screen come on when you press it anyways? Unless you've got it locked? Or is that not the case on the N900 (that's how the old N8x0's worked)
2) Danger would be having the phone in your pant pocket and moving too fast/quickly would cause screen to come on as well which would be bad (the phone doesn't know when you "pick it up", it only knows "when it's been moved with a certain amount of force in a certain direction"...

As I said I don't have one to test with but... thoughts?