Just spoke to Apple and apparently if you install the MCE OptiBay yourself (or a battery for that matter) you void your warranty as they are not user serviceable upgrades. Memory and hard drives, in the hard drive slot, however, are user serviceable upgrades and will not void your warranty.
If you want to maintain your warranty and install a 2nd hard drive in the optical drive bay you can but you have to have it installed by a certified Mac service center. This is a ridiculous and expensive policy, given how simple the install is.
If you want to maintain your warranty and install a 2nd hard drive in the optical drive bay you can but you have to have it installed by a certified Mac service center. This is a ridiculous and expensive policy, given how simple the install is.