Just a quick note about performance and Battery Life. NOTE: this was in Mountain Lion Beta things may have changed since the official release.
My battery life drop by 50% and performance decreased by 20% or so... So in short just keep in mind that you are going to experience some performance hits hopefully in the Official Release it isn't as big of a hit as I experinced also to note I do access large amounts of data, such as Aperture, and Parallels running 2-3 VM's, which increased my CPU usage by 25-30% due to encryption.
Anyways just wanted let you all know my experience with File Vault
Thanks for reading