Currently Being ModeratedFeb 27, 2013 10:49 AM (in response to jonnylikes2rant)
Welcome to the Apple Support Communities
It depends. See > http://support.apple.com/kb/PH11302
You have ordered a Late 2012 iMac, that has got Thunderbolt ports. If your wife has got a Mid 2011 or newer MacBook Air, you can use your iMac as an external display for the MacBook Air, because the MacBook Air has got a Thunderbolt port. However, if it's an older MacBook Air, you won't be able to use it because you can't connect a MacBook Air with Mini DisplayPort into another Mac with Thunderbolt to be used as a display.
If your wife has got a MacBook Air with a Thunderbolt port, just buy a Thunderbolt cable like this one and follow the steps of my link above to set up your iMac as a display.
If the MacBook Air has got Mini DisplayPort, you can still use the iMac as a display if you install a VNC client as ScreenRecycler on both computers. This application uses your network to use your iMac as a display for the MacBook Air and it works great, but it completely depends of the network