On the vanilla 6600's, the sensor reading is disabled in the BIOS by default. The sensor is there in the chip, but if you want to see it, you'll have to modify/flash the BIOS. You'll have to decide if you're willing to do that to see the temps. Instructions can be found here:
You don't even have to use a hex editor anymore, since NiBiTor has a simple checkbox for it now.
I believe credit for this trick goes to those over at www.mvktech.net