I've been using the istat pro widget for a time now and notice the GPU Diode runs aprroximately 6 deg lower than the GPU Heatsink. Is this normal?
Nothing to worry about.
Not sure how confident I feel about cracking it open and giving it a dust.
The biggest problem with doing that for many users is getting dust trapped behind the glass during reassembly, or getting fingerprints on it. It's a good idea to move the glass panel far from the iMac if you open it to dust it out & to vacuum the area after blowing the dust out with compressed air & letting it settle. Using a vacuum inside the iMac is not a good idea (because of possible static buildup & because you might damage something with the nozzle or brush if you aren't careful).
It isn't really that much like dusting furniture because the exposed electronics are much more delicate than furniture. Also, it takes a lot of dust to materially affect cooling so you probably won't see much difference if you try it. Personally, I think once every 3 or 4 years is more than often enough to prevent any overheating problems, unless maybe your iMac lives in a very dusty location.
Just thought all those helpful people out there (more helpful than Apple Store in Cardiff, Wales) that the fault is still on my intel !mac 24inch.
Even after a visit to said store and they testing the computer for hours (so they say) the fault did not replicate itself. They returned it to factory settings in case it was a software problem.
Within two hours of it being on at home the screen went light grey. It seems to be getting quite warm on the left side and the GPU Diode is running at least 7 deg lower that the GPU Heatsink now.
Did the Apple Store test the unit while running the same applications you normally use? If so, then there may be something related to your home environment. Do you have the computer in an area where it gets good air circulation? Not in an enclosed cabinet, surrounded by books, etc? Is your home warmer than the store? Is the computer near a heater vent or radiator? Try moving your computer to a different location in your house for a short time or taking it to a friends to use it as you normally would to test.
If it continues to overheat in other areas, talk to the Apple Store again, see if they can make arrangements for you to use it (maybe at an out of the way shelf space) for a bit to try to replicate the problem for them to look at it.