Here's a quote from OWC on the non-Retinal models:
While the both the new Macbook Airs and the MacBook Pro with Retina Display have new, custom connectors for their SSDs, the updated 2012 13” and 15” MacBook Pros continue to use industry standard SATA drives for their storage. That’s great news for upgrading; it means you can install an OWC Mercury Electra 6G or Mercury EXTREME Pro 6G SSD in the main drive bay and get the amazing performance boost that comes from an OWC Solid State Drive.
There’s also an optical drive in these models. It’s obvious Apple is moving away from the optical drive as a media source; they’ve removed it completely from both the Air and now the 2012 MBP Retina models. You can do the same with the standard MacBook Pros, except you can put something even more useful in its place: more storage! The OWC Data Doubler, allows you install any 9mm or narrower 2.5” SSD or platter-based drive in the optical bay, and fits without issue in the 2012 MacBook Pros.
You may recall the problems people had using 6Gb/s SSDs in the optical bay of some 2011 MacBook Pros, so we did some preliminary tests this time around, too. We’re happy to say that the 15” model can take a 6Gb/s drive in either bay. Unfortunately, we can’t recommend using a 6Gb/s drive in the optical bay of the 13” model at this time. In our testing, we found the 6Gb/s performance of the bay to be very poor. The 3Gb/s models work with no problems, though, so you can still put a high-capacity platter-based drive in the optical bay and put the super-fast 6G SSD in the main drive bay, where it’ll work just fine.
We will be reviewing options to resolve the problem in the 13″ 6Gb/s optical bay , just as we are continue to do so with the similar issue affecting the 2011 MacBook Pro 15″/17″ models.