Apple Tech support
I just had an interesting conversation with an apple tech support rep. When I inquired about the target display mode, he put me on hold for a few minutes and came back on the line with a senior hardware engineer. The engineer informed me that they have been receiving numerous questions about target display mode. He said at this time there is only capability for TDM via thunderbolt from other thunderbolt equipped devices (however he could not confirm that this would apply to both the 27" and 21.5" iMacs, like Macworld reported). However, he said this is something they are "aggressively" looking into and hope to be able to release an adapter (either directly through Apple or via a 3rd party manufacturer) that will bring capability for 3rd party devices to take advantage of the feature once again.