updated 09:52 am EDT, Wed September 25, 2013
CPU on 21.5-inch model no longer replaceable
Both the 21.5- and 27-inch models of the new iMac have undocumented secondary PCIe slots, suitable for an SSD or a Fusion Drive, according to an iFixit teardown. The extra slots appear regardless of whether or not the Mac was ordered with an SSD or a Fusion Drive in the first place. That stands in contrast with last year's iMac configurations.
The teardown also shows a union of SATA power/data hard drive cables, and a slimmer CPU heat sink. That CPU, however, is now soldered to the logic board on the 21.5-inch system. This prevents owners from upgrading or replacing the processor on their own, although the iMac was already a difficult computer to upgrade. iFixit does note that the CPU can still be replaced on the 27-inch model.