It can be done, but I’m not sure which SKUs it applies to, or that it’s particularly advisable.
Both the 2.9 GHz and the 3.1 GHz 21.5″ iMacs have space for a second drive bay, and there’s an iFixit tutorial explaining how to install a second drive. (iFixit usually tear down new hardware within a day or so of release, and have a lot of tutorials for things like hardware upgrades and repairs).
Currently their page for the 27″ model doesn’t have such a tutorial. However, there’s a page on their forums where people are discussing associated teardowns and the possibility of such an upgrade. The poster ifixit611 writes:
The 27″ iMac holds one 3.5″ drive (unlike the 21″, which uses a 2.5″ drive), and one “blade” SSD like the new MacBook Pros use. Unlike the new 21″ iMac, you can upgrade the RAM on the 27″ w/o cutting the machine open (there’s an access hatch on the back).
Replacing the disk requires cutting open the display and removing the left speaker. You’ll need a proper mounting bracket (e.g., an Icy Dock) to mount a 2.5″ SSD in place of the 3.5″ drive. Adding/replacing the blade SSD requires pulling out the main logic board.
They conclude that they won’t be performing the upgrade before iFixit or OWC provide a proper “kit”.
So it sounds like, in theory, you could perform this upgrade on a 27″ iMac, and the instructions from iFixit for upgrading the 21″ would probably be helpful in this regard. That said, hardware is not my area of expertise and I generally get twitchy about opening up iMacs. Do at your own risk, etc.