Here's a little more info on the mode..
http://gamer.blorge.com/2008/10/27/still-need-left-4-dead-with-call-of-duty-5-zombie-mode/
Gotta say, it does sound good... but a purchase over left 4 dead? Not for me.. (even though I think I'll be getting CoD5 andyway) left 4 deads game play and AI surpasses the zombie mode by leagues. In cod5 you have zombies that shamble and tear some barricades down.. in left 4 dead you have a full on invasion of 28 weeks later style zombies trying their fucking damnest to eat you
More action packed and adrenalin fueled that the nazi zombies.. not to mention you get some awsome weapons.. where's the chainsaw in cod5? nope maybeyou can use the garrand? oh you're dead lol
Either way, I'll be getting Left for dead and i'll be buying cod5 regardless