the 27" iMac has a mini displayport connector that can be used either as input or output. Currently all other Macs with a mini displayport connector are output-only.
Yes, theoretically you could connect a DVR's output to your 27" iMac with the appropriate cable & adapter, although I'm not aware of any that exist yet for this purpose. Unfortunately Apple hasn't yet published any details about driving this input from sources other than another Mac with mini displayport output, so I'm not sure what display resolutions and formats would be supported, but hopefully solutions will soon be available.
does your DVR have a Firewire output? if so, that's another possible way to input HDTV from a cable box, I know it works for the Motorola HDTV cable box I have, I can record HDTV programs on my MacBook Pro.