When I bring my work laptop home, it would be convenient to use my i5 as an external display ...
... Connect a male-to-male Mini DisplayPort cable to the Mini DisplayPort on each computer. The 27-inch iMac will enter Target Display Mode and display content from the source computer.
Note: If you are connecting two 27-inch iMacs, connect a Mini DisplayPort cable to each computer and press Command F2 on the 27-inch iMac keyboard that you will use as an external display....My work laptop has a Display port output, but not a Mini DisplayPort. Alas, modern video cabling is a complete mess. (Yes, it's all about the DRM. Oh, for the brief shining moment of VGA everywhere)
It looks like what I need is a DisplayPort to Mini DisplayPort adapter cable ($12.95). I'm going to give this one a try and update this post with my experiences.
Incidentally, Belkin sells a $30 Mini to Mini cable through the Apple store and circuit assembly sells a $13 version.
I found a quick topic review on Ars Technica as well.