Good news will notch this down as an option for Audio/Video Labs with the 27inch iMacs.
We often have users wanting more screen real estate when using the MBPros on certain projects.

Glad you were the Guinea pig as it does not come cheep on a whim.

Well I'm not so happy this morning as it's not working. I just get a black screen on the iMac. If I unplug it the iMac goes back to its main screen so it's obviously getting some sort of signal... just not the correct one or some such.

Not sure where the fault is, but I'm certainly underwhelmed by the quality of the build of the Atlona device.

I am unsure but i know that "Target Display Mode" is very picky with the way things are done unlike when you plug and play other external displays.. Maybe try and follow the instructions/process to the letter and I hope the adapter is not a dud.

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

Yeah, sadly those instructions are just "plug it in" ... ! It was working fine last night, and a little this morning. But then it went and now won't come back.

I've seen a lot of discussion of people reporting the black screen when in TDM, have tried usual things, reset PRAM and SMC and so on... no dice.

I know it's not the MBP as my 20" cinema works fine, and I'm pretty sure it's not the iMac as nothing has changed and it comes out of the black screen OK when disconnected, so it's either the Atlona or the cable (which is the Apple advised Belkin.)