After Apple changed the hard drive in my iMac they left dust under the glass. I live in the country and didn't want to take the 60 mile round trip lugging a 27" iMac. So ifixit to the rescue!
Smooth as glass. Attached the suction cups per the ifixit instructions. Pulled the glass off, blew out the dust, replaced the glass and DONE!
Nope. Couldn't have been any easier.