Using iMac as Mac Pro display I currently have a iMac (27-inch, Late 2009, 3.06 GHz Intel Core 2 Duo) which I would like to use as display for a new Mac Pro.  How do I do that?
You can use the MiniDisplay Port on the back of the iMac and put the iMac into target display mode, turning it into an external display.

More info: http://support.apple.com/kb/HT3924