Hi,
As one of the latest victims of the "replacement cycle" (various issues: yellow, dead pixels etc,) with the new imacs and on number 4 at the moment, I recently took matters in to my own hands re:the "dust behind the glass" and once you have pried that glass off and cleaned it (and the bare lcd panel) you will hit yourself in the head for making such a big deal out if it.
It really is *incredibly* easy to take the glass off (suction cup from top) and wipe these surfaces. Considering how common this problem is, and how easy it is to "fix", I am surprised it's not in the "getting started" booklet.
For suction cups, I simply borrowed some from my shower radio but any decent sized suction cup would likely work (e.g., some toy from the dollar store).
If your screen is otherwise perfect and your hardrive is quiet, do yourself a favour and don't send your iMac back for such an easy fix!
-Iamthinking
Maybe Apple should be simply adding suction cups to the package?
Tom B.