The iMac's just scratch extremely easy.
I've owned 2 iMac G5 systems (which I no longer have).
I found that the finish on both of them were extremely fragile.
Just touching them would leave permanent marks.
So, just the action of setting it on your desk, plugging it in, and tilting it to the angle you like will cause scratches or damage to the surface.
These usually only show-up in the bright light, or when you look at it from an angle.
But, if you are really wanting to keep it looking nice, then whatever you do, Don't clean it - ever.
I used a water-dampened lint-free cloth to wipe the surface, and found that it left lots of light scratches.
So, that's just part of owning one of Apple's high-gloss devices. They will be scratched.
I don't like it, but it's been that way for quite a while now. Sucks, but that's the way it will be until they change to a more durable finish.