I guess I've hit the limit of calibrating my iMac's screen.
I was working on a landscape shot and had it open in Preview on the left side of my screen, and PS on the right side. Before any adjustments the Preview image looked duller (less contrast and less saturation). Weird, I thought. Then I reversed the windows' positions... The PS image now looked dull !
I turned of all the lights - no difference !
I turned my head around (maybe my left and right eye work in different colour spaces ) : apart from getting dizzy, I didn't notice much of a difference.
Now for the really scary part : my wife noticed the difference as well.

What's going on here ? Is it just a poor quality display ? Anyone noticing something similar ?
Now to be clear, I can prevent any light from reaching my screen, in a completely dark room, so this is not some ambient light problem; I've also tried different color synch profiles; I've put my screen in another room... still the same.

I think you've answered your question. Those displays are not really designed nor manufactured to be ICC calibrated reference monitors. Many advanced amatuers I know of use them, but when I hear them suggested in the pro circles, I just cringe.

absolutely. What I'd like to know is if anyone else has a similar experience - if I'm alone I'll probably raise the issue with Apple.

How have you calibrated and profiled these two screens?

There's only one screen, calibrated with Colorvision's Spyder; sorry if my initial post was unclear, what I'm comparing is the left and right side of a single screen (hence my surprise). I am actually seeing a similar efect using the factory profile.

Ok, now I get it. Time to call Apple. You should not see huge variations across the screen. Instead of using Photoshop and Preview you should make a duplicate image or multiple duplicates and place them around the screen in Photoshop to pin this down further before climbing the Apple hill.

This may be a stupid question, but you don't happen to have a subwoofer or other large magnet near your monitor, do you? Magnets can cause colour shifts on a monitor...

Mike.

That's a good suggestion - though here it didn't help when I tried.
I'll give Apple a call
thanks to all for your input

Definitely call AppleCare. From the conversations I have had with them, the iMac has the same screen as the Cinema Displays. Good quality display for the money.

Have you checked the file information for both pictures. One might have defaulted to a lesser resolution