I just bought an xbox 360 slim console after getting the fabled red ring of death on my last 360, and I had audio and video fed through my Pioneer reciever. On the old box, there was never any connectivity issue; the reciever would recognize the console and it would play just fine through HDMI. I tried connecting the slim today, using the exact same wires and connections, and I'm getting zero sound and picture. I tried using a different HDMI cable (which works with my PS3 and cable box), I tried using the included standard definition cables, and still I get zero picture.
To test a potential hardware failure, I used the SD cables and hooked the console up to an old Magnavox I have. It turned right on and worked perfectly. And since the reciever is sending and recieving pictures and sound perfectly (including through the xbox HDMI input; I rerouted cable to go through there and it works fine), the obvious fact is that the 360 slim is not "speaking the same language" as my reciever. And the display reset gimmick doesn't do squat. I held down the Y and R Trigger for five minutes and nothing happened.