Works great. I bought the Belkin cable (Mini Display Port to Mini Display Port) when I picked up my 27" iMac. I plugged on end into my MBP and the other into the iMac, and the iMac instantly became a secondary monitor for the MBP (the iMac was on already). Clicking CMD + F3 on the iMac toggles between the iMac display, and the MBP display.
I can set the iMac display to be an extension of the MBP desktop, or I can mirror it. If I mirror it, the iMac resolution decreases to match the MBP. If I leave it as an extension, the iMac stays at full resolution. I can also move the menu bar to the iMacs screen if I want.