My iMac has one of those, a little fine dot inside the screen. It's either some crud that was inside the tube when it was built that's decided to stick somewhere, a manufacturing defect in the mask, a manufacturing defect in the phosphor triads, or something similar along those lines.
And basically - they don't go away. If it's the first option, shaking the screen about might dislodge it - but there's no telling whether that'll make more muck fall about inside the display, or just make it move about to another location.
Dana