I am having this exact problem. It's frustrating that Apple doesn't seem to want anyone using an iMac with a non-mac computer. I was told at an apple store that it is possible to use 2011 iMacs as a monitor, that there was an adapter but they were out of stock. He said it was a belkin brand but I can't now recall what it was. I suppose if Macs only work with other Macs the way an adapter would have to work is to imitate the signal from another apple using thunderbolt, probably too sophisticated to be feasible.
Starsandtripe, how is the latency with Remote Desktop? I would've thought it would be unusable for almost all applications that require realtime response. Do you notice much lag? And how is the quality?
Additionally, are you aware whether or not it's possible to use the Remote Desktop for mac as a second monitor rather than a single monitor or perhaps a mirror of the desktop?
In other words, is it possible to have a normal, PC compatible monitor as display #1 and a mac working as display #2? This is key, as I need to be looking at many windows at once.