What am I going to do with an iMac 27 inch's display 3, 4 years from now?
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/