I have been using my 2010 iMac 27" as an external display from a windows laptop. Since the recent update, I can no longer receive the signal on the mini-display port. Have software updates prevented this even for older Macs?
It was excellent for using my work laptop at home.
Now, when I plug the laptop into the iMac, it recognizes there is a display, "tries" to show the display, and then reverts to Mac OS 10.7.5.
there's a lot of misinformation so far here. Yes you can use all the 27-inch iMacs as external displays.
However there a few problems.
2009-2010 27-inch iMacs can only work with Apple computers that have the Mini-DisplayPort.
2011-2012 21-inch and 27-inch iMacs can only work as a display with Macs that have thunderbolt ports.
so as long as your 2011 MBA has thunderbolt, the new one will do fine. your 2010 MBP will not be able to use the iMac as a display.