There may be an option in the my.cnf file to select which address mysql listens on (but I'm not aware of one).
As mossy mentioned, you may want to throw up a firewall.
Also, mysql has its own security about which users can connect to which hosts. See
http://www.mysql.com/doc/en/GRANT.html
It differentiates localhost users from users on other hosts. But application security isn't as good as just not listening on a public address..