I would love to see zombies .. and since Jordi said he wanted to try and get them in by November, I am wondering why this question?

On how they should spawn, to limit the crash/lag potential, why not base them the number of ACTIVE members of a tribe?

That way, for tribes with2-10 members they will have an reasonable amount of zombies to have to deal with, and areas with no active players wont have any spawn.

Anyway, if they will spawn on junk piles and head straight for inhabited tribes with active players it will be more like what you would expect if this was .. um... real.