There is a fair sized thread on here devoted to the light spots on the screen and many people have them in the same place. I can't say I know for sure what caused mine, but one is just below where the power button is when the slider is open and I have noticed if I'm not careful pressing the button with the slider open there is colour distortion in that exact spot caused by pressure on the back of the screen, and the other larger one is exactly where my finger hits the screen when I scroll web pages, could be a coincidence though. Some people have speculated they are caused by certain components behind the screen getting hot which would explain why they are often in the same place for different people. I do have a very small barely noticeable spot on my spare N900 as well, but it is in a different place.