That should be rather easy and in fact it's working without issues (I'm using at at several sites as here in germany ISPs have the habbit to force disconnects every 24h. >:(
Configure your DynDNS at services>DynDNS. Make sure your pfSense has the real public IP at WAN and it not natted behind another router.
Then follow http://faq.pfsense.org/index.php?action=artikel&cat=10&id=41&artlang=en