New iMac's come with a film wrap that covers the entire glass display and which extend to the back of the aluminum enclosure. It's usually pulled so nice and tight at the factory that it may look like it belongs. Did you remove that plastic film when you purchased the system? Look at the back of the system and see if you can find the edge of the film. It should be fairly noticeable. The film also covers the intake vents on the bottom of the iMac so it's important to make sure you remove the plastic film if it's present.
If the plastic film has already been removed then you may have some residue left behind from cleaning. If you have cleaned the display what kind of cleaning cloth did you use? The iMac cleaning instructions state that you should only use water to dampen a soft, lint free cloth.
http://support.apple.com/kb/HT3226#desktops
When cleaning the outside of your iMac, first shut down your iMac and unplug the power cord along with other connected devices. Then use a damp, soft, lint-free cloth to clean the computer’s exterior. Avoid getting moisture in any openings. Do not spray liquid directly on the computer. Do not use aerosol sprays, solvents, or abrasives that might damage the finish.
To clean the display on your iMac, first shut down your iMac and unplug the power cord along with other connected devices. Next, dampen the cloth that came with your iMac—or another clean, soft, lint-free cloth—with water only, and then wipe the screen. Do not spray liquid directly on the screen.