By spots do you mean "pixels" or "areas that are a bit darker than the rest of the screen"? If the answer is pixels, then they're probably dead pixels as m_stolle said. If they're areas, then its probably been there all along and they're caused by the unequal distribution of light from the LEDs that make up the backlight (I assume the n770 uses a LED backlight, never seen a good pic of a disassembled one). In a review for the n770, there was mention of dark spots, which looked like they were caused by the backlight. Theres not much you can do about it, unless you take it apart and add more LEDs. Other devices have the same problem, my PSP has darker areas along the bottom of the screen where the LEDs are, usually only visibly on solid, light colored backgrounds, even then its not a problem.

And magnets do nothing to LCDs, I've set magnets right on the screen of my PSP and Game Boys. Its still not a great idea, as they could possibly screw something else up.