Can I connect the iPad 2 video out to an iMac?

This is not a AirDisplay question, it's about the HDMI connection.

I don't have a TV. I use my main computer, a 27" iMac, for watching movies.

The iPad 2 has HDMI out and games can output to 1080p while using the iPad as a controller, as seen in the recent RealRacing HD announcement.

Does anybody know if it will be possible at all to use an iMac screen, and its mini display port, in place of HDMI out?

Yes you can do it using an Active HDMI to MiniDisplay port adaptor like this but as you can see it's not cheap

On the page edude05 linked, there is a "Did You Notice?" scroll which, frankly, I had not noticed before:

Did You Notice?

The Mini DisplayPort connector on the 27-inch iMac supports input as well as output, so you can connect to any external source that has HDMI output and use your iMac as a display.

So this implies that you can route video from the iPad through its HDMI output into the Mini DisplayPort on the iMac.

But if that's the case, why are the reviewers disparaging the 27"?