Mine wasn't dust. Was kinda difficult to wipe off. Almost reminded me of water stains which I guess could be possible if condensation got in between the panels.
Seeing as the mass bulk of my problem was right over the heat sink it does make me wonder....
Thanks, Oggy for posting this - would have never tried this on my own without the pictures and walkthrough, and now that I'm done (sigh of relief) my iMac feels shiny and new again.
My smudges/dust streaks beneath the LCD panel were TERRIBLE. All over the screen. Incredibly distracting when photo editing - I kept trying to photoshop out tones and smudges that weren't in my pictures. This is a serious, serious failure on Apple's part to adequately seal the LCD panel...
Quick note: for what it's worth, the 'pry points' didn't necessarily hold much weight with me... I found it easier to just get a very skinny flathead screwdriver wedged between the outer frame and black plastic (per the instructions) - if you just do it a little bit all around and 'loosen' it, that seems to work well before lifting up.
Also, be sure to spend a LOT of time cleaning the underside of the LCD adequately - you do not want to have to do this again if you see one last smudge. Having a light nearby to shine across the screen helps to see smudges and dust.
Finally, the majority of dust was actually on the silver part beneath the LCD for me - I believe this is the backlight? Don't know what it is, but that is where the major dust streaks and stains were... it literally looked like someone had poured dirty water on the panel, let it dried, and then reassembled it. Ridiculous. Microfiber cloth + iKlear did the trick on all - just go slow, and be careful.
I wish I'd taken before/after pictures to post - but needless to say, my screen is once again clear. I'm a mac-ed out guy, but this has been my first major 'record-scratch' in their products. I didn't get Applecare because I thought, "it's a desktop... what can go wrong?' Big mistake.
Good luck to anyone else who tries this - but for whatever it's worth, it works. Just take your time. And THANK YOU OGGY.