What is weird is I am running an HTPC which has DVI out on it. I have a DVI to HDMI cable going from HTPC to the reciever. From there I have HMDI to DVI adapter connected to 50ft DVI cable (chose DVI because it was MUCH cheaper at this length) with another DVI to HDMI adapter going into the HDMI input on the back of the TV (65" Mitsubishi Diamind series DLP). This setup works great when running HTPC! No lag, holds a beautiful picture, etc.
Its the microphone in rock band thats giving me the headache. Rock band seems to be okay for the rest of the instruments but the microphone audio is what lags. For instance, if I tap on the mic I hear it out of the speakers almost a full second later. Pretty much ruins the singing portion of the game...
Also noteworthy, I have a 50ft usb extension running back to the closet connected to the 360. I use this connection to connect all the instruments to the usb hub that came with the game in the front room below the TV. Not sure if this would cause that but as I said, the other instruments seem to play just fine...