Using an iMac as a secondary monitor I have an early 2009 iMac (The last one that was 24 inches) and a Macbook Pro from work. Occasionally I work from home and would like to use my iMac as a second monitor with the Macbook.

Is this possible? And what cable do I need? If not, I'm thinking of getting the new non-retina iMac, is it possible with that?
As explained here, the first iMac to allow this is the 27-inch Late 2009 iMac.

There are, though, some Applications which may achieve something similar, requiring both computers to be connected to the same LAN/WiFi/VPN, and sending all the information, the one to be displayed on the second computer working as a display, through it. Such as ScreenRecycler. You can find some more here.