Is he hinting Zombie Siege?
I believe not. Zombie Seige is an entire new gamemode so I doubt Notchy would release both Adventure Mode and Zombie seige at the same time. Zombies are quite common anyway.
Next update:
1.7 - Pistons, Sticky Pistons, Shears
1.8 (A long time away) - Better coloured lighting, new mob, ravines underground, NPC villages and a whole lot of other stuff that notch claims to be keeping as secret as possible but is slowly revealing it via Twitter and Tumblr