Sorry to hear the news. Mine started last week with one single white dot and now I have 8 with a couple black dots.
Do you have a warranty? If you do, take advantage of it because from what I understand a new light engine, which is what you'll need, is very costly. I would predict them to be between $750 and $1000. I do not know yet, because I am under warranty and the tech. is going to order it. I will probably know within a week or so.
If you do not have a warranty, then you are SOL. You could attempt to fix it. If I were forced to fix mine, I would of course start by trying to find a light engine for cheap. Then its a pretty simple process. From what I understand, the light engine sort of slides out, and the new one slides in. You could attempt to change out the "DMD chip" that Samsung owners have done with their white spots on their DLP sets. That would be more labor intensive although cheaper. I have read that those who changed out "DMD chips" also had picture quality issues.
Anyways, I am sorry about this for you, and Mitsubishi basically SUCKS.