New iMac SSD upgrade?

We don't know for sure on the 27" since nobody has torn one down yet as far as I know.

If it's like the 21.5" then you'll have to use a heat gun to loosen the screen glue, and then you may have to re-apply some adhesive or double-sided table to re-attach the screen when you're done.

Edit: iFixit's teardown shows the foam adhesive pretty well:
(Step 5)
http://www.ifixit.com/Teardown/iMac+Intel+21.5-Inch+EMC+2544+Teardown/11936/1

No teardowns of the 27" yet but im almost 99% sure it will be the same as the 21". All you need is a guitar pick to break the double sided adhesive, someone on here has already done the SSD upgrade on the 21" without any issues. It is advised that you wait till someone finds out exactly what double sided tape Apple are using so you can restore it back to factory specs for warranty issues if needed.

Im doing the same with my 27" but i ordered the fusion drive, i will use the 128GB blade SSD for my bootcamp install and a 512GB 830 Samsung SSD for my OSX and Apps. I will use the 3TB drive i pull out of the iMac in a USB3 external enclosure i have for my media files.