this support document states that the 21.5 and 27-inch 2011 iMacs can be used as external displays via thunderbolt. the document also clearly states that the 27-inch 2011 iMac cannot be used as an external display via Mini Display port. the document does not say anything about using the 211.5-inch 2011 iMac as an external display via MiniDisplay port. can the 21.5-inch 2011 iMac be used as an external display via Mini DisplayPort?
iMac, OS X Mountain Lion, use iMac as a Mac Mini monitor