Apple likes to use "blade style" SSDs instead of the conventional (but space-wasting) SSDs built inside a disk-sized box. Early pictures from the insides of the 21.5" iMac reveal space for one of these blade style SSDs.
But not every 21.5" iMac has the connector loaded.
iFixit took apart the lowest-price (US $1299) iMac, and there was no SSD connector (it would be where the red rectangle is drawn):
http://guide-images.ifixit.net/igi/ZTSiyt32BumalDcI.medium
But a Japanese site posted pictures (and words I can't read) showing the SSD connector installed:
http://www.kodawarisan.com/k2012_02/images/113022.png
http://www.kodawarisan.com/k2012_02/archives/2012/11/aa_imac_aeaaa_1.html
Mixed in with the Japanese words, I see "iMac 21.5" and "2.9 GHz", which suggest to me that the pictures came from the US $1499 model.
So it seems to depend on which model of 21.5" iMac you get.
The hard drive in a 21.5" system is a standard form-factor 2.5" drive. There will likely be ways to replace that with a 2.5" SSD. The 27" iMac is expected to be similar, except the drive is most likely a 3.5" one. There's not likely a second conventional hard drive "slot" in either iMac, since Apple doesn't offer any 2-drive options.
(On some Apple systems, the flash memory is just soldered onto the logic board, saving even more space. But I don't expect that to be the case on the 2012 iMacs. We won't be sure until someone opens up a 27" model.)