The N900 has a proximity sensor.

As I'm aware of it, it's only used during a conversation to detect the ear near the screen.

I'm thinking of filing an enhancement/making brainstrom.

When the N900 has been
1) laying down (up or down faced)
2) standing on stand
for at least 30 sec (a configured value by user)

This should not be active when in the upright position (eg. in pocket)

Then when I swipe my hand in front of then screen, the "swipe to unlock" screen should appear.

The reason for this is, when during the night I wave my hand over the sensor I want to see the time.

What do you think?
Any caveats? (power consumption / accidental usage / ...)

Not feasible because of energy consumption (if you want the sensor to be constantly polled).

Its not a proximity sensor, its a light sensor. It knows its dark near your ear and so turns the screen off.

Your hand waving idea wouldn't work if it was dark because it wouldnt be able to "sense" your hand.