I enabled the full disk encryption or at least I think so. The HD shows up as "Encrypted Logical Partition" in Disk Utility.
When I boot up the macbook however I can choose between logging in as Andreas Bonini - with my user account password - or entering the disk password.
If I choose the former option, I am never asked for the disk password.
This is not what I had in mind -- I wanted something like TrueCrypt, where everything on the disk is encrypted and the only way to access it is entering the disk password; the macbook shouldn't even be able to know what user accounts if any are present before unlocking the disk.
My user account password is obviously considerably less secure than the disk password.