I managed to setup an encrypted partition that's mounted on boot using dm-crypt/LUKS.
The relevant entry from my /etc/fstab:
/dev/mapper/st_crypt /media/st ext4 defaults 0 2
The partition is mounted at boot, and I can write to it as root just fine, but I have no idea how to make it writable by a normal user (i.e the users group). Any suggestions?