What am I going to do with an iMac 27 inch's display 3, 4 years from now? I want to buy a 27 inch iMac, but the monitor cannot be re-used... it cannot be hooked up to any KVM (Keyboard / Video / Mouse) switch to be used with a PC too...

Also, 3, 4 years down the road, I know the iMac will become slow and probably need to buy another computer.  At that time, what am I going to do with the monitor?  Apple is saying they save the "microwatt" between your typing, but what about wasting a good monitor like that?
The display port on the latest iMac's (actually since late 2009) acts as both an output (for an external display) or an input (to make the imac screen a monitor).

See http://smokingapples.com/asides/new-27-imac-doubles-up-as-external-monitor/