I realize I'm a little late to the party here... but I have a late 2012 iMac that the top glass cracked on, but the underlying screen itself was perfectly fine.
I ended up buying a replacement glass+screen and after I'd successfully repaired my iMac I was left with a working screen glued to a broken pane of glass. I can confirm that you can relatively easily separate the two with dental floss, baby oil and a razor blade. Contrary to popular belief, the two pieces are not fused together using UV glue, but merely stuck together with a non-curing (doesn't harden) sticky silicone based adhesive. Much like a lot of cell phone screens. after separating the two, you can clean off the residual goop from the screen with baby oil (if you didn't know, baby oil is REALLY good at eating through adhesives) and a credit card. resist the temptation to use a razor or anything metal because the screen is not made of glass and will scratch.
Once you remove the silicone goop, (there's a little over a quarter cup of it on the screen) you can clean off the oily residue with glass cleaner and paper towels.
Fun fact: The glass panel and the screen are the EXACT same parts used in the 2011 model iMac, but some rocket scientist at Apple had the brilliant idea to glue them together so you have to buy a whole new screen if the $15 glass panel gets damaged.
so... now I have a perfectly good extra 21" 1080P LED screen. I'm considering trying to find the shell of another 21.5" iMac and hacking together my own 21.5" Cinema Display that never was.
