Knowing less about iptables than smb.conf and based on what I found in the existing /etc/sysconfig/iptables, I added these two lines:
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 137:139 -j ACCEPT -A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 445 -j ACCEPT
Are these the entries I _should_ add to iptables?
I ended up with the following on my samba PDC: -A RH-Firewall-1-INPUT -p udp -m udp --dport 137:138 -j ACCEPT -A RH-Firewall-1-INPUT -p udp -m udp --sport 137:138 -j ACCEPT -A RH-Firewall-1-INPUT -p tcp -m tcp --dport 139 -j ACCEPT