I've had them repaired on sensors of various cameras a few times. Basically all they do is map out the dead pixels in the camera's software (I guess it's like telling the camera those pixels don't exist, so ignore them). Once the mapping is done the faulty pixels don't feature anymore and you don't see them (or their absence) in the final picture. This, as you can imagine, can be done any number of times. After all, you have millions of pixels on the sensor and you won't miss a few that disappear. Nikon did this for me free of charge, I guess since it does not involve physically working on the camera.

Good luck.

My Nikonians gallery