Display external video on iMac
Is it possible to connect external video to an iMac?


Yes, the latest 27" iMac can act as a monitor via Mini DisplayPort. From Apple:


  Mini DisplayPort output with support for DVI, VGA, and dual-link DVI (adapters sold separately); 27-inch models also support input from external DisplayPort sources (adapters sold separately)


You'll need an adapter to convert your laptop output to Mini DisplayPort. Be careful, as some adapters, such as those provided by Apple, are designed to output DVI/VGA/etc, and aren't capable of being reversed to accept it as input.

further reading: http://support.apple.com/kb/HT3924