It looks like there is possible to mount an own SSD now? didn't the last iMac require some kind of Apple supplied mountingkit which you only got when you bought the SSD option?
Yes, it SEEMS that another hard drive can now be fit under the optical drive bay without such an adapter. However, these mounting holes you can see on iFixIt are still not the standard way to attach harddrives, so some sort of adapter might still be required.
On a sidenote, when I was talking to the (very friendly) Apple store employee on the phone yesterday to order my 27" iMac, I was asking the very same question. He put me on hold to talk to a tech-guy about this issue, and came up with the response that an Apple service partner will be able to fit in an additional SSD later. I'm not too certain that this will be true, maybe he just misunderstood my question and meant that the service partner can replace the current harddisk with an SSD (instead of adding an additional one), so I'll remain sceptical.
But the iFixIt-teardown has given me hope that it might in fact be easier now to add another harddrive without BTO.
