I bought an iMac from the local Apple store in June.
"About this mac" tells me it's an iMac9,1 Intel core 2 duo 2.93 Ghz 4GB RAM. The screen casing is aluminium and I think the the screen size is 24inch (it is running 1920 x 1200 screen resolution if that gives a clue)
I have a photo of the crack here:
http://www.inperspective.com/temp/crack.jpg
The crack does not appear to be on the exterior glass. I can run my fingernail or even the corner of a piece of paper across the screen surface where the crack appears and I cannot feel any bumping - it appears to be totally smooth on the outside.
I'm pretty sure it is a crack not a hair because looking at it from the side you can see the light defracting along the crack line.
Hope this helps
Many thanks for your response.