You might check to make sure that the cable/sat box has "HDMI out" selected in the menu (if it's not auto detected) and this would be in the cable box's settings menu, or changing the resolution on the box may help also.
It's also possible that your cables aren't rated HDMI v. 1.3. Do you know if they are?
Keep me posted.
I have a brand new 40" Samsung LN40B500. When I connect the TV with the Cablevision Scientific Atlanta cable box using HDMI cable, the picture shakes slightly - but noticeably. I have tried 2 HDMI cables, but results are the same. When I connect to the cable box using component cable, there is no issue. I would like to find out if it is the TV's HDMI input that is the cause of the problem or the cable box so that I can exchange it if necessary. But, since Cable box is the only HDMI capable device I have, I don't have a way of testing this out. Anyone else has experienced this issue with these Samsung TVs?