Is it possible to use a 21" iMac as an external monitor from a MacBook Pro via Thunderbolt? I'd like to get an external monitor for my 15" MacBook Pro, but the 27" Cinema Display is too large (and too expensive) for me.  Ideally I'd like a 21" version of the Thunderbolt Cinema Display but such a beast does not exist.

Is it possible to use a 21" iMac as an external monitor from a MacBook Pro via Thunderbolt?

Thanks.
The answer is sometimes yes. I found a list of iMacs that support "Target Display Mode" here ...

http://support.apple.com/kb/HT3924#1

... and it includes "iMac (21.5-inch, Mid 2011)" and "iMac (21.5-inch, Late 2012)".

That would imply that any iMac with a Thunderbolt port can be used in Target Display Mode.