I've noticed the same problem with my 970 and the hi-def cable box. I'm running component from the box to the TV and optical from the cable box to the 970. I also have the analog stereo output from the cable box running to the TV also. With both the TV audio and 970 audio playing there is a noticable delay through the 970. I know my lip sync is zero and my distances match what I measure.

The only config where I see this problem is when I'm using the optical inputs on the 970 (not too often). Maybe there's some extra delay to decode the digital optical? (I wouldn't be very suprised.)