From anandtech:
Both of the 21.5 and 27-inch iMacs use SO-DIMMs, however the 27-inch model does allow for end user upgrades. Remove the power cable and there's a button that will pop out a little panel giving you access to the system's SO-DIMM slots. The 21.5-inch model needs to be taken apart to gain access to the memory slots unfortunately.
http://www.anandtech.com/show/6402/up-close-with-the-new-27inch-imacs-user-serviceable-memory-panel
So is as replaceable as the hard disk was, hard for some people but doable.
Both of the 21.5 and 27-inch iMacs use SO-DIMMs, however the 27-inch model does allow for end user upgrades. Remove the power cable and there's a button that will pop out a little panel giving you access to the system's SO-DIMM slots. The 21.5-inch model needs to be taken apart to gain access to the memory slots unfortunately.
http://www.anandtech.com/show/6402/up-close-with-the-new-27inch-imacs-user-serviceable-memory-panel
So is as replaceable as the hard disk was, hard for some people but doable.