HDMI must be HDCP Compliant
I just bought a Pioneer VSX-72TXV -- one reason being the HDMI in-out as well as upscaling -- not to mention a s**tpot full of other great features.
One requirement of this unit is that any digital video source (HDMI or DVI)must be HDCP compliant -- which I'm OK with since I don't plan to pirate any films, etc. BUT, although I assumed that Comcast would be "in tune" with the whole HDCP compliance thing in their STB's I was wrong. I tried every combination of settings I could think of, asked the cable guy (how was here for another problem) and nothing would make the digital video from the STB display via HDMI to the HDTV. I called Pioneer Elite user support (which I must say is absolutely top notch) and they said the HDCP thing is probably the problem. So I called Comcast and, unfortunately, at least in my area the Comcast Motorola STB with DVI out is not (yet) HDCP compliant. So I can't use the HDMI pass through on the receiver quite yet -- must wait a few months for Comcast to upgrades their HDTV STB.
Long story -- and I'm not sure if that's your problem, but I suspect it is.
BUT -- in the meantime I am using the VSX-72 to convert component video (it's analog, so not subject to HDCP complaince issues) from two sources -- the STB and DVD -- to HDMI output to my Panny 42" HDTV (1080i) and it looks great.