Deadline reported on the news, and was picked up by /Film. The reason for the change is most likely because of its rival movie, Sherlock holmes: A Game of Shadows. Both films are aimed at young males (Sherlock because of the new action-packed take on it) which would negate the chance of those two films sharing an opening weekend audience. It's new release date is December 21.
Another reason it may have moved forward is Alvin and the Chipmunks: Chip-Wrecked, which could draw families that may take their little boys to see Mission Impossible. It puts it directly against David Fincher's The Girl with The Dragon Tattoo, so Paramount must believe that that film will bring in a different type of audience. But two days later on the 23rd, it is pitted against a number of films being released, i.e Tintin, the Darkest Hour and We Bought A Zoo.
Anyway, Mission: Impossible Ghost Protocol is released December 21st starring Tom Cruise, Jeremy Renner and Simon Pegg!