Just bought a N900 on ebay that seams to suffer from this problem.
I have tested the script and in direct sunligt the script returns Closed.

Is this a reason to get the phone replaced? Any who have got the phone replaced due to this problem?

Thanks to this thread my proximity sensor is much better.
I have used black insulation tape to cover the bottom portion of the lower part of the sensor. Similar to the picture up-thread.

I have yet to try strong sunlight (it's a rarity here) but testing with a 100w light bulb shows great improvement.