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Scientists Use WiFi to See Through People's Walls
(www.popularmechanics.com)
This is a most excellent place for technology news and articles.
You are correct, at best this requires some a priori knowledge of the room. You can kind of do basic motion detection blindly though. They are just measuring the channel response via the 802.11 preambles, so for basic presence detection knowing that the channel response is changing is enough.