From Apple iMac Hands On - iMac - Gizmodo:

The iMac has a new trick, too—its DisplayPort can turn it into a second screen, receiving video (and audio, pending availability of 3rd party adapters) input from DVD players or a MacBook. Apple said it was HDCP compliant so it should be fine for watching Blu-rays on, via the port, via a separate player.

This is something I've wanted since the first iMac. Your monitor is one of the longest-living pieces of hardware you can buy, second only perhaps to a keyboard or mouse (post-USB), if you like them well enough. Buying a great monitor is certainly the best way to stretch your money. Putting the cash in up-front makes for years of increased real estate across a number of computers.

Now this is still a fairly expensive monitor, even after subtracting the value of the iMac innards, but at least your purchase doesn't lose its value nearly as quickly as the old model. 17" G4 lamp iMacs are going for $100 on eBay now at times, and that monitor and stand alone is worth at least that much, I'd think, if you could hook a Mini to it. Even if this is, as Gizmodo give or take suggests, a way of getting BluRay coming through your iMac, now you'll be able to grab a Mac Mini in three years instead of tossing out the whole contraption.

So the iMac is your next Mini's (or Windows tower's) monitor. And the 27" can be mounted on the wall. Perhaps it's your next TV. Ah, the beauty of input ports.