According to the preview article at photo.net it is based on the sensor in the 400D/Rebel XTi but is improved:
"Although it is based on the image sensor used in the EOS Digital Rebel XTi, the EOS 40D Digital SLR's 10.1-megapixel CMOS APS-C size image sensor has been significantly improved thanks to the use of larger microlenses over each pixel to reduce noise and expand sensitivity up to ISO 3200. "
http://photo.net/equipment/canon/40D/Sounds like kind of a hybrid between the 400D sensor and the electronics improvements of the 1D mark III.