I personally setup Llama to turn the wifi when I'm not connected to a tower that I previously defined as "wifi available". It's nifty because I never have to explicitly turn wifi off then back on during my travels. To add a new wifi zone, I simply add the cell tower to the previous list and let the app do the rest.

Llama by itself uses less battery than leaving the wifi chip on, even in passive scanning mode, as the phone is always looking and checking cell towers.