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http://www.hiddentelevision.com/samsungledmirrortv.htm or http://www.hiddentelevision.com/instructions.htm for the DIY version
It tends to make it not work as a mirror. I have seen this done with a large bathroom mirror though were a small amount in an unused corner is removed to have your screen show there, while the rest is still available as a mirror. This is likely what I will be doing now that I have the programming instructions from this 'ible
Table Saw Cross Cut/Miter Sled Combo