This issue was actually discussed in length here, and one user reported a problem with a non-Samsung television.
The key is to unplug the cable box.
I'm sorry you got the runaround, but three out of three people on the forums here noted that unplugging the cable box fixed the problem, and the issue was not limited to Samsung products.
I have a Scientific Atlanta HDTV Explorer 8240HDC cable box from Time Warner Cable, which I just picked up a few days ago when I purchased a Samsung LN40A650 LCD tv. When I connected this cable box to the tv, a number 2 surrounded by a black shaded box appeared on the upper-right side of the screen on every channel. I tried using my Monster Ultra 1000 HDMI cable and the composite cables provided by TW cable, but I was unable to get rid of the number 2 and shaded box.
I then called Samsung and they told me that this was a "known issue" and that there is no fix at this time. I then asked if this was just a problem with the Samsung 650 series. I was told that this issue had been reported for the 550, 650, and 750 series.
In light of this information, I returned by 650 and purchased the LN40A630. Unfortunately, the same exact problem occurred when I hooked up the 630 to the cable box.
When I returned the 630 to the store, the manager said that he had never heard of this problem occurring before, and that he had sold over 1,200 630s and 650s, so the problem MUST be with the cable box.
When I called Samsung back, they said that they couldn't guarantee that a different cable box would resolve the problem, but that I could try doing this to see if it would work. I simply can't continue to run around town buying expensive tvs to "see" if I can get them to work with my cable box. Most stores are charging a 15% restocking fee for opened boxes, and I don't feel I should have to take the risk of spending $140 to "see" if the Samsung tv I buy will work with a different cable box.
Please let me know what I can do about this. Should I simply stay away from Samsung all together and look at the Sony XBR6?