I have an 811 and had the same problem.
While viewing a channel where you have this problem...
Go to the 811's system menu (I am at work and trying to remember)
I think it options 9, 1, and then 6 for HDTV setup.
Anyhoo, the target menu is the one where you can set the type of OTA antenae, the aspect ratio of the display device and the resolution (1080i, 720p, etc.).
On the right there's an option to "Adjust" select this. It will then eventually show you the channel with a menu overlay that allows you to move the picture up/down/right/left. Once you are happy, click the enter button on the remote to save the setup, and you're done.
The problem with this is:
1) Not all HD channels have this problem
2) For me after moving the picture up to hide the white bar on top, on some channels, I can see a faint white band on the bottom.
3) For some reason, sometimes I have to go in and resave the adjustment to get it to work - this may be related to 1 above.
I had this same problem with my Samsung 160 OTA receiver and fixed it in the same way. I hope that as HDTV gets going and mature, this will be less common.