Seems like apple themselves had a good solution to this with one model of the iMac that supported use as an secondary monitor. Imagine if every laptop / iMac supported use as a secondary monitor.
Yeah, to be honest, I would be happy if I could buy another product such as the mini and then plug it into the display, but sadly not supported, there are some third party ways of doing this, but I am not sure they are so great.
I actually use my iMac with my windows laptop for work, by plugging in a video capture device via USB and going via HDMI to the laptop, it works okay, but i'm not sure i'd want to run a shiny new Mac this way.
