Here is my solution for Linux distributions using Upstart 1.4 and higher.
Save the contents below into a configuration file under
/etc/init, for example:
# Dropbox upstart script for user db_user, db_user can be replaced by anything you like.
description "Dropbox db_user"
start on runlevel 
stop on runlevel [!2345]
# The next two lines are optional. I use these to be able to share the Dropbox using Samba. Within group shareusers are all Samba users.
# setgid shareusers
# umask 0002
# The next line is necessary to be able to synchronise files with non-ASCII characters.
To start up the daemon without rebooting:
sudo start db_user
Note: By uncommenting the setgid line it is not necessary to set the setgid flag on the
.dropbox.cache folder as indicated by other sources on the internet.