Rubenerd

If you’re a *nix nerd like me, you most likely formatted your Mac with the non-standard case sensitive variant of HFS+ (ala Mac OS Extended), which FileVault doth protest too much about if you try to enable it on an existing account.

Fortunately there’s a workaround: simply create a new user account and click the "Turn on FileVault protection" checkbox. If you’re installing OS X from scratch, its simply a matter of creating a temporary account with the installer, using it to create your real account, then deleting the temporary one.