It's a stupid design, but no, you can only PIP the internal tuner source. If your cable box supports outputting over coax and HDMI at the same time, you could hook up your cable box to both and get PIP functionality (though sadly, I think most cable boxes only do one output at a time).
You can read through a big thread on the this very PIP complaint with the next model up (the 650); all of Samsung's 2008 LCDs have the same limitation: http://forums.cnet.com/5208-13973_102-0.html?forumID=146&threadID=314854&messageID=2898865#2898865
So, with the LN40a550, you can PIP a TV channel with another video source. Sounds simple right? But see my issue is, I do not use the cable TV port thing, I have a HDMI Digital cable box, I get my TV channels from that source.
When I click the good 'ol PIP button it displays that SD cable jack image, but since I have nothing plugged into that port I just get a static box.
Is there a way I can set the PIP to be the HDMI source and not the cable jack one?

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