Can I use a 24-inch iMac as an external display?

[MacTemplario]

I have a 24-inch iMac that is ripe for replacement; my wife barely uses a 2010 15" MacBook Pro.

In 2011 I'm thinking of buying a MacBook Air for her and I take the MBP.
-I would buy a 27-inch display for the MBP;
-ideally she could use the 24-inch as external display for the MacBook Air
-or, alternatively, I would use the 24-inch as external display for the MBP

Would any of these solutions work?

 

[old-wiz]

the 24" iMac does not have video in.
You would have to use something like screen recycler or
one of the Elgato eyetv devices.