I've been meaning to do some battery tests with these things for a long time. An xBee can sleep at nearly no power use until it gets a wakeup signal. I'm not sure what the power usage of the PIR sensor is, but we'll find out.
In full on mode it works fine, but that will run the battery down fairly quickly. I'd like to set a periodic wakeup once an hour or so to just report in battery voltage or at least to give a ping that it's still there and of course it needs to wakeup whenever the PIR fires.
So now off to study the sleep settings.