Other World Computing reports installing an aftermarket SSD in the new 27-inch iMac is not an easy task because you'll be needing a special mounting bracket and connector cables that are only supplied with the expensive Apple SSD. It's not impossible, but there's not really a graceful way to upgrade the new iMac with a SSD:
The SSD would go underneath that metal lattice and circuit board, and would sit in its carrier against the back wall of the iMac. Connections would be made with a special power cable that would power both, and the data cable would run to the third SATA channel on the logic board.
Of course, you can still install that second drive on a non-SSD iMac if you:
1. Connect a data cable to the empty SATA port on the back of the logic board (a process requiring the removal of several screws and partial removal of the logic board.
2. Get a Y-cable that goes from 1 SATA male power connector to 2 SATA female power connectors. (This step is actually pretty easy.)
3. Find a way to secure your 2.5″ drive in the space that the carrier would normally hold it.