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There are I believe some kludges you can do - if you can get your video stream output via FireWire or ethernet, then you should be able to find solutions that will let you display that video stream on the iMac, but that isn't really using the iMac as a monitor (and if you want a gaming PC the latency would presumably make it completely unsuitable).
Basically, as far as I'm aware, the answer is no.
the iMacs can output video but cannot take a video input
The current 27" iMacs actually can be used as a monitor.
Mini DisplayPort.my emphasis
The Mini DisplayPort lets you connect an external display, including the Apple LED Cinema Display, to your iMac. On the 27-inch iMac, the same port offers input, too. So you can connect any external source that has DisplayPort output — including a MacBook or MacBook Pro — and use your iMac as a display.