To expand -- my thought on chef proper is that if you are using chef, you probably want to use chef to maintain the baseline for servers, independently of Atlas. Therefore, Chef proper will probably be a provisioner which looks for available servers by querying on tag/role/etc.
It may make sense to also do an initializer/installer pay which can add additional elements to the selected node, but I doubt we (Ning) will use that if created. It makes sense to me to have it, though.
-Brian