Is there any way to remove the backpart of a iMac 27"

This might solve your problem: http://store.apple.com/us/product/MC772Z...

The direction state you must tilt the screen all the way forward, use the provided card to depress the inner switch, and while the inner switch is depressed you are able to pull the base stand out of the imac/display. The base stand will still be connected to a spring load holder which is connected to the inside the iMac/display but will now be protruding out from the iMac/display. This spring loaded arm holder is what you will connect your vesa mount to after removing the stand that is connected to it.

IT IS CRITICAL THAT YOU MAKE SURE THE SPRING LOADED ARM HOLDER PROTRUDES OUT OF THE IMAC/DISPLAY AND LOCKS INTO THAT POSITION BEFORE REMOVING THE BASE STAND. YOU WILL KNOW THAT IT IS LOCKED WHEN YOU REMOVE THE CARD USED TO DEPRESS THE SWITCH AND THE BASE STAND CANNOT SWIVEL BACK AND FOURTH. IF YOU FAIL TO EXTERNALLY LOCK THE SPRING LOADED ARM HOLDER AFTER YOU UNSCREW THE BASE STAND FROM THE ARM HOLDER, THE ARM HOLDER WILL DEPRESS BACK INSIDE THE IMAC/DISPLAY AND IT IS NEAR IMPOSSIBLE TO PULL BACK OUT. (IF THIS HAPPENS, YOU WILL HAVE TO BRING YOUR IMAC/DISPLAY TO THE NEAREST APPLE STORE AND HOPE THEY HAVE THE PROPER TOOL TO PULL THE INNER ARM HOLDER BACK OUT. THIS IS NO EASY TASK, EVEN FOR THE APPLE "GENIUSES."

Here's a video on removing it. It's in German but it's a video: http://www.youtube.com/watch?v=onN1rrB9L...