Check Reasonable's thread:
From Digital Foundry article Pg 3:
"So if many of Kinect's limitations are software in nature and can be engineered, or "evolved" around, what of the hardware itself? Perhaps the biggest surprise - and disappointment - in viewing the final Kinect spec up against the PrimeSense reference camera is that the resolution of the all-important depth map has been reduced to just 25 per cent of its original size: 320x240 up against 640x480."
I'd assume this is based on a hands on with the device.
Edit: Also on Digital Foundry they have an article from 30th June that confirm the source of their info was Play.com:
They also say:
"While the information is unconfirmed via official channels, Play said that the details are direct from the manufacturer, while specs collected by Digital Foundry but not published to date tie in extremely closely with the new data. The smart money is on this being the real deal."
So it looks like they've had some experience with the device. DF are usually fairly good with this sort of thing aren't they? I know Kotaku stuff needs to be taken with a pinch of salt.