There are many step-by-step guides on the internet that explain how to add an SSD to an existing Mac, and create a 'Fusion Drive' that has the speed of an SSD, but also the capacity of a Hard Drive. All these guides fall short in one way that was important to me.
Creating the Fusion Drive the way these walkthroughs say (including OWC's exceptional guides), destroys the Recovery Partition that exists on the drive. Without a Recovery Partition, you cannot enable FileVault2, and will need some other external boot drive if you ever need to perform maintenance on your internal drives. For a laptop computer that might be far from home, not having a Recovery Partition was unacceptable to me. Also note that if you buy a Mac from Apple today with Fusion Drive, it DOES come with a Recovery Partition, so it is indeed possible to do.
It turns out that Apple's Core Storage technology is more flexible than these walkthroughs give on. You can enroll an individual partition of a drive in a Fusion Drive, instead of the whole drive. This means that you can join just a specific data partition of your HD with an SSD, and leave the Recovery Partition intact.
I've created a Fusion Drive (SSD + Original HD), that HAS a Recovery Partition, and is now encrypted with FileVault2. It is a lot faster than the original, so I'm calling it a success.
Steps to reproduce:
diskutil listat the prompt.
Mac OS X Hints