Got my N900 barely used second hand (Seller said it was in perfect condition - it wasn't, but it was close enough that I didn't mind), and I had two of these spots in the screen from the get-go. Actually, it may have been just one at first followed by a second sometime soon after. I haven't really been bothered by them though.

nokiauser14: I'd recommend insisting on a replacement device if it's a problem. Or having them replace the LCD screen with a new one. Because the problem is hardware-linked, and since it's physically in the screen itself that's pretty much the only way you'd get rid of it.