What is the actual resolution of Kinect's depth sensor?

[JaggedSac]

I have read several articles stating that the Kinect depth sensor is  is 320x240, but all the specs I have seen, have it at 640x480.

Microsoft's Store:

http://store.microsoft.com/microsoft/Kinect-for-Xbox-360/product/C737B081

• Color VGA Motion Camera 640 x 480 pixel resolution at 30FPS
• Depth Camera 640 x 480 pixel resolution at 30FPS
• Array of 4 microphones supporting single speaker voice recognition

Amazon:

http://www.amazon.com/Kinect-Sensor-Adventures-Xbox-360/dp/B002BSA298/ref=sr_tr_2?ie=UTF8&s=videogames&qid=1281529239&sr=8-2

• The game "Kinect Adventures!" will be included with the sensor.
• Color VGA Motion Camera: 640x480 pixel resolution @30FPS
• Depth Camera: 640x480 pixel resolution @30FPS
• Array of 4 microphones supporting single speaker voice recognition
• Fully compatible with all Xbox 360 models

Can someone point me to the source detailing the reduced resolution please?