RAM was easily upgradable on the late 2011 21.5" iMac. (It had four user-accessible RAM slots.)
It isn't on the 2012 model. From the iFixit teardown, it appears that there are now only two slots, and you have to crack the case, "unglue your screen, and remove the logic board" to get to them.
As a practical matter, this means that if you think you will ever want 16 GB of RAM (e.g., to extend the future life of the machine), you may want to custom-order 16 GB at purchase time. Or you may want to consider a 27" iMac, whose RAM is still user-upgradable, allowing for a much cheaper upgrade ($50 instead of $200) using a third-party 8 GB RAM module.
Thanks Tom, I think whatever model I choose, I should have 16 GB ram anyway.