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I'd like to think that there would be an iSight built into the displays since Apple seems to no longer produce the original iSight. The only questions I have with an iSight built into the display is what if you're running dual monitors? Would you be able to disable one camera for video chats?
 
I'd guess if the ACD did have an iSight built in then you would have a USB connection along with the DVI connection to plug into your MBP.

If you are using 2 monitors, then you would only plug in one USB plug, allowing you to choose between the two.
 
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