It looks like a pressure from the back problem. If you look at it while sliding, you can notice an increase in spot intensity. Nothing to do about that else than disassembling and try to reassemble better. On my experimental device I did it, on the 'production' one I live with the spots

Oh my, I have those spots too. Just disassembled my phone to re-attach my USB-port and didn't pay attention when I put everything back together. The upper part with the display didn't fit that well and after I tightened the screws I could see the spot in the middle flashing when sliding the keyboard up and down. After a while I figured out how to assemble the N900 properly and everything went back to normal: white spot, barely visible.
When I push the display (it's frame) down and slide the keyboard up and down I can see the spot flashing again. I assume that it has been like that since I got the phone and the two white spots are from pressure on the display, either due to bad assembling from Nokia or due to a construction failure. The latter wouldn't really surprise me

Why can't manufacturers take old models and improve them instead of producing something completely new with new faults all the time?