Ask iFixIt: the thinner Apple makes their products, the less user serviceable they are. That’s why the new 21.5-inch iMac doesn’t allow users to upgrade the RAM themselves. The 27-inch, though, apparently does, a consequence of the added space.
John Brownlee is a Contributing Editor. He has also written for Wired, Playboy, Boing Boing, Popular Mechanics, VentureBeat, and Gizmodo. He lives in Boston with his wife and two parakeets. You can follow him here on Twitter.