Good overview on Session Border Controllers -- essentially, they are needed to make VoIP work across firewalls and between NAT'd devices. Makes sense that they would essentially be integrated with firewalls.
It looks as though SBC technology would be best applied inside other networking gear, like firewalls and switches, it shouldn't surprise anyone when existing network equipment companies begin to buy the SBC vendors. Always On Network (Rafe Needleman): VoIP: Less Easy Than it Looks