With the multiplayer engine, the mode is capable of having twice the amount of zombies and players (says 3arc) without too much lag added in.
It will probably also be an extra feature if you are tired of 4-player.
Like Playlists > Zombies > 4 Player Survival, 8 Player Survival, etc.
Its has been confirmed you can play 8 players, and don't forget its up to 8, you can still play as 2,3,4,5,6 in private match, but yeah i couldnt imagine 8 player zombies on a past map as there are only 4 windows, and therefore killstealing would become a massive problem, and that even happens now, however if work is put on new maps it will be good.
Since Black Ops 2 is now running on the multiplayer system, lag will probably be reduced by alot. And also, since I am always getting invites to zombies matches, I don't like to exclude my friends certain matches just because there isn't enough room for them. I think this is good to a certain extent. 8 player would be too long to make it to high rounds. I would still only play 2 player if I'm trying to make a round record.
Very valid points have been put forward in this discussion so far. Enough good points that I have changed my mind. I very much agree about being able to have more than 3 friends playing with you at once but what about matches with randoms?
Anyway, I change my vote so...