Playing survival mode as special infected kind of defeats the purpose, IMHO. But some guys at Left 4 Dead mods say they'll drop configs for that soon - if Valve doesn't spoil the fun first.
In the video above, posted Wednesday, you see human controlled infected, plus 8 bots versus human survivors, playing on The Last Stand. Of course the point of survival mode is to see how long the survivors last against an unstoppable horde. That doesn't mean taking over the zombies extends your life any, unless you like hiding in the trees instead of bum-rushing the set.
You see some hunters and smokers go down super fast, so the modders bring out the Tank - outfitted with the Tourette's Guy soundpack, in fact - and he's still aubdued in 30 seconds of toe-to-toe combat.
Ultimately, the survivors are going to die, and with this mod, the bragging rights go to the human-controlled zombie who finishes off the last one. Assuming the bots don't do the job first.
The modders, Hunterbrute and Naru, are trying to perfect their work before releasing it. "That is, if Valve doesn't see this and patches it." Check back to their page for updates.
Survival Mode Becomes Versus Mode [L4D Mods, thanks Stephan B.]