An LCD image will have millions of little dots called pixels. They are constantly changing colors to produce your image. When an LCD pixel dies it will end up looking like a dark little spot on the screen, if it dies off. This to me is the least offensive, but it can die on in RED, GREEN, or BLUE. Man do I hate that. Once you see it you can't take your eyes off from it. There is no fix for it once you get it, in fact most warranties don't even cover it. I have actually seen manuals that will tell you this is normal. It does still happen, and in my experience to often. They have improved, since its not near as bad as it was in the past, but as long as the warranties do not cover it I will take no part of it. In fact my local Circuit City has a very large selection of LCD's, and last time I was there I counted 3 of there sets with dead colored pixels.
DLP's can get something similar if one of their little mirrors stops functioning. This will give you a small dark pixel spot. I have read about a few of those, but I have yet to actually see one and I would suspect it is less common for a dlp to have a faulty pixel vs an LCD.