dead pixels or, in your case, hot pixels, are a byproduct of manufacturing a sensor with 16 million photosites in such a small area (each 'pixel' is actually 1R, 1B, and 2G pixels in a square). You can't get them all perfect all the time. some may be perpetually dark, but others are stuck bright. There's usually an upper limit on the number that can be defective before they throwout the sensor, but many sensors, from any given production run, will have these tiny imperfections. what pixel mapping does is takes a picture with no light (it can just keep the shutter closed) and looks for these aberrations, and stores their location so it knows to subtract them from any further photos.
Incidentally if you let your sensor get hot by doing a lot of live view/long exposures, more hot pixels can show up (but will usually go away again when the sensor cools down)
Last edited by jerm1386; 10-26-2011 at 05:39 PM.
Reason: I was wrong on the number of photosites