Actually, the update script runs regardles of the IP change every week on Sunday early morning. There is a cron job set up in /etc/cron.weekly called refresh-ddns.
This is causing problems with my dyndns.org account. They consider it as an abuse if the IP record for domain updates more frequently then once in 28 days (if there is no change in IP address). Recenly one of my domains was blocked for this reason.