Automatic mounting of encrypted partitions on OS X
I'm using Unlock by Justin Ridgewell to mount my FileVault2 (CoreStorage) encrypted HDD that I have replaced my SuperDrive with.

Unlock will loop through your CoreStorage encrypted volumes (sans boot) and ask if you want them unlocked upon booting.  The passphrase for unlocking them will then be stored in the System Keychain so that another user could login and have them automatically mounted even if they don't know the passphrase for the volume.

So it's a boot time unlocking vs a user login unlocking.