We made this known earlier in the release post for the new iMacs, but it may not have been clear to many of you: Apple's new 27-inch all-in-one Macs do in fact have two drive bays one for hard drives (HDs) Serial ATA and one for solid state drives (SSDs).
None of the four pre-configured models by Apple offer SSDs as standard (all have 1TB hard drives with the exception of the cheapest, which is 500GB), but customizing the machine you have four options in addition to the original, check out:
It is possible to exchange the HD of the iMac for a 1TB or to discard the HD and simply put a 256GB SSD. But the coolest (and the big news) are the last two options, available for 27-inch models: you can keep your HD (1TB or 2TB) and * add * a 256GB SSD. Expensive, but it is the perfect combination for those looking for performance * and * high data storage capacity.
Alternatively, it is quite possible that users can install an additional SSD later when purchasing an iMac with only one hard drive.
(via 9 to 5 Mac)