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Basically, no.

You could theoretically use a piece of DVR-type hardware like the eyeTV to pass a signal through to the iMac, but the resolution would be lower than the screen's native and there's usually enough lag with a system like that to make gaming awkward--even a half-second can badly screw up most games (where it makes no difference with TV).

Plus it's ~$100 of hardware; you can get a 20" monitor for not much more than that.
 
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