Yeah nothing is gained from saying he'll be in it then not having him show up, unless it's Moffat's plan to show a surprise regeneration, but by the time the 50th airs surely they'll have already been filming (if not completed) the Christmas special, so it's highly unlikely they could keep it a secret that Capaldi had done a substantial amount of filming.
I really hope Smith does get the send off he deserves, he deserves to be the star of his last episode (at least until the final few minutes) even though it would be interesting if perhaps the regen was 2/3 or 3/4 of the way in and the newly regenerated, and somewhat below par, 12th doctor had to resolve the situation.