Hi djaygray,
I am having the same problem. I absolutely love my 27" iMac, but it makes me feel sick about 5 minutes after using it. The problem has become worse, because when I first purchased it about a year ago I had no problems using it hours at a time.
Looking around at different posts, it seems others are having trouble with the backlit display.
So far I have played around with the color temperature settings and the brightness. Nothing helps. I have ordered some Gunnar glasses to see if they will cut down on the glare and help. Failing that, I am going to have to switch to a pro machine where I can buy my own display. I have two 24 inch screens at work side by side, same distance from my chair, and they cause me no problems whatsoever.
I am really bummed out about potentially having to buy a new screen or change computers. Part of the reason why I love the mac is that that it does not have wires all over the place. I ***** that I will have to go back to a cabled monitor...but it beats the alternative of feeling sick all of the time.
I sure hope Apple looks at some different screen technology in the future.
It took me a long time to figure out that it was just this screen and not other health issues like an inner ear probelm or something. It is definitely the screen.
Pleaee let me know if you find any resolution to the issue (no pun intended). I will post if the Gunnar glasses help.