I too experienced problems immediately after getting my 24" iMac:
https://forums.macrumors.com/threads/342644/
I mentioned it to a friend of mine, and he was able to properly diagnose the problem. I had the computer too close. The sensation I experienced around my eyes was exactly like trying bring your hand into focus when its about an inch from your face - it felt like I was forcing my eyes wide open all the time. He pointed me at this page:
http://www.office-ergo.com/viewing.htm
The source of my discomfort was because of 'Convergence' - "when the eyes turn inward toward the nose when we view close objects."
Why did I have the LCD so close?? The dpi on the 24" imac is a LOT higher than the displays I'm used to using, so the print is much smaller. Because the print is so small, I kept bringing the display closer in so I could read it.
I also think the display is too bright. I've got it down all the way and its still too bright for me. Check out this application, it might help:
http://www.charcoaldesign.co.uk/shades
I'm hoping Leopard will allow you to make the display even dimmer than Tiger, but probably not

I can understand how there is a limit to a display's brightness, but I don't see why you can't make it dimmer.