I have the following line in /etc/sysconfig/iptables

"-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 5432 -j ACCEPT"

I restart the firewall and then run nmap on the port that's supposed to be open, why is it still showing up as closed? Is there something I'm missing?

[root@wookie root]# nmap -p 5432 localhost

Starting nmap 3.48 ( http://www.insecure.org/nmap/ ) at 2004-04-05 13:36 CDT
Interesting ports on wookie (127.0.0.1):
PORT STATE SERVICE
5432/tcp closed postgres

Nmap run completed -- 1 IP address (1 host up) scanned in 0.397 seconds
[root@wookie root]#

One thing to keep in mind is that nmaping localhost can give you inaccurate scan results. If you scan from another system, do you still see the same reuslts (the app may listen on your network IP rather than the loopback). If you temporarily shut off the firewall and then do the scan, does it still report it as closed? If it's still closed, then it's not your firewall and is likely a network problem or the application isn't running properly.

If it reports it as open when you turned off the firewall, how did you add that rule? Directly editing the /etc/sysconfig/iptables file can cause problems. Also make sure that you have a line in your iptables rules that allows connections in the ESTABLISHED,RELATED state as well. If both those are OK, then post a copy of the entire iptables file or the output of iptables -L -v (make sure to remove any identifiable ip addresses).

[root@wookie root]# iptables -L -v
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
11 1121 RH-Firewall-1-INPUT all -- any any anywhere anywhere

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
0 0 RH-Firewall-1-INPUT all -- any any anywhere anywhere

Chain OUTPUT (policy ACCEPT 13 packets, 928 bytes)
pkts bytes target prot opt in out source destination

Chain RH-Firewall-1-INPUT (2 references)
pkts bytes target prot opt in out source destination
0 0 ACCEPT all -- lo any anywhere anywhere
0 0 ACCEPT icmp -- any any anywhere anywhere icmp any
0 0 ACCEPT ipv6-crypt-- any any anywhere anywhere
' 0 0 ACCEPT ipv6-auth-- any any anywhere anywhere
9 828 ACCEPT all -- any any anywhere anywhere state RELATED,ESTABLISHED
0 0 ACCEPT tcp -- any any anywhere anywhere

state NEW tcp dptostgres
0 0 ACCEPT tcp -- any any anywhere anywhere

state NEW tcp dpt:ssh
0 0 ACCEPT tcp -- any any anywhere anywhere state NEW tcp dpt:http
0 0 ACCEPT tcp -- any any anywhere anywhere state NEW tcp dpts:netbios-ns:netbios-ssn
2 293 REJECT all -- any any anywhere anywhere reject-with icmp-host-prohibited

if the port is CLOSED instead of FIREWALLED, it often says there's no service working on the port.

So maybe the PostgreSQL service that's supposed to be accepting connections on that port is configured incorrectly?

Firewall looks ok. Did you try temporarily stoppping the firewall:
service iptables stop

Then nmap or use one of those free internet sites that will do a nmap or nmap-like scan. I know www.grc.com and sygate both offer those services free.

Also check the output of netstat -pantu. Specifically look for a PostgreSQL entry on port 5432.