Connect a HDMI enabled PC to a new 2013 iMac 27" to use as monitor?

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The Late 2012 iMac has only got a Thunderbolt port, and it can only be used as an external display for other Macs with Thunderbolt > http://support.apple.com/kb/PH11302

That means that you shouldn't be able to use your iMac as an external display for another PC, more when the PC has got a HDMI port. If you want, you can try buying a Mini DisplayPort to HDMI adapter and a HDMI cable (or a Mini DisplayPort to HDMI cable) and see if you can put your iMac in Target Display, but do it at your own risk because it's probable that it won't work

Using the mini-DisplayPort to HDMI adapter in reverse will not work. Using an HDMI signal as input to a Thunderbolt port would require circuitry to change the signal, and while there are such adapters for Macs with mini-DisplayPort inputs, I've yet to see one that works with iMacs with Thunderbolt ports.

Regards.

You would need a video input device to capture the HDMI signal and appropriate software to display it. Since you can pick up 27" LCD monitors for under $250 and 24" LCD monitors for less than $150, I'm not sure it would be worth all that effort unless you just don't have space for another display.

Regards.