I posted this in NBR but I wanted to share the love here as well. It benefited me a lot since I don't have a need for an optical drive and my SSD is only 80 GB.
This is the optical bay I bought... SATA Hard Drive Caddy Bay Macbook PRO 13" Unibody A1278 - eBay (item 260760330830 end time May-28-11 02:09:19 PDT)
You do not have to buy the same optical HDD caddy but it does have to be 9.5mm in height. Anything bigger will not fit.
Do this at your own risk. I take no responsibility for any mishaps you encounter along the way.
Before you do this, install your SSD as you would normally, and install windows from the DVD with the optical disc drive still in the M14x. IF NOT THEN YOUR COMPUTER WILL BOOT FROM THE HARD DRIVE YOU HAVE WINDOWS OS ON REGARDLESS WHICH SATA SLOT IT IS IN.
Step 1. Optical Bay I Purchase from eBay
Step 2. Opening the bottom panel
Step 3. This is the DVD optical disc bay that comes in the M14x
Step 4. Removal of screws. (GO TO STEP 5 BEFORE REMOVING STRIP)
Step 5. Careful removal of the SATA strip
Step 6. Optical disc bay vs Optical HDD bay
Step 7. How to remove the SATA strip from optical disc bay
Step 8. The SATA strip
Step 9. Removing the caddy from the optical disc drive
Step 10. Attaching the strip on the optical HDD bay exaclty as it was on the optical disc bay.
Step 11. The optical HDD bay with strip attached.
Step 12. The gap filler provided with the optical bay. It's to fill the space between the HDD and the optical HDD bay.
Step 13. What the gap filler looks like with the HDD in the optical HDD bay
Step 14. Removal of black panel form optical HDD bay. * must be removed for it to fit inside the M14x *
Step 15. Optical HDD bay with black panel removed.
Step 16. Placement of optical HDD bay inside M14x
Step 17. Drivers automatically installed.
Step 18. Computer detecting secondary hard drive
Step 19. HWinfo with the both the SSD the HDD in the info (Sata II 3Gb/s)
I will fix it up and add any additional pictures if needed as time goes on. If you don't like the guide, please don't criticize. This is for the visual people out there who need help and want to enjoy the additional hard drive.
If you have any questions, please feel free to ask