Target Display Mode with an iMac
In our 2011 iMac review I mentioned that you could use the new iMac as an external display if, and only if, you had a Thunderbolt cable connecting it to a 2011 MacBook Pro. When I reviewed the iMac, you couldn't buy a Thunderbolt cable. Now you can.
The process is pretty simple. Just connect any 2011 MacBook Pro to any 2011 iMac via a Thunderbolt cable. Hit Cmd + F2 on the iMac's keyboard and boom, your brand new iMac is now a secondary display for your MacBook Pro. Note that you need a modern Apple keyboard for this to work. Older Apple keyboards and some non-Apple keyboards won't send the right key code and will refuse to activate target display mode on the iMac.
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The iMac has to be fully powered on when this happens. You do lose all keyboard/mouse support on the iMac, however the machine doesn't go to sleep when in this mode - it'll remain on, although likely in a low power state. Any applications you have open on the iMac will remain open. Any music playing on the iMac will continue to play and you don't get use of the iMac's speakers from your MacBook Pro.
15-inch MacBook Pro (2011) + 27-inch iMac (2011) running in Target Display Mode
For 2011, this is admittedly an odd usage model. I'm not sure how many people want to connect their brand new MacBook Pro to a brand new iMac and use the latter as just a display. Down the road I can see this being a good feature as it lets you upgrade to faster MacBook Pros and still use your iMac as a larger display if you'd like.
I would prefer it if the iMac allowed itself to boot into a minimal environment where only the display and necessary ICs were powered, leaving the CPU, GPU, hard drive and the rest of the system mostly powered down. I suspect Thunderbolt complicates this as the controller needs a number of components powered up before it'll function. Perhaps future versions of the iMac can find a way around this limitation however.
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