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You can use any of a number of EyeTV products for this purpose. They supposedly work very well and can also be used to turn your Mac into a DVR for your TV.
 
I think someone should start a thread listing all the methods to use an iMac as a screen (ScreenRecycler, TV input devices etc) and stickie it. It seems to me that this question gets asked on an almost weekly basis.
 
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