FORWARD chain and Interfaces

[netfilter@buglecreek.com]

I have a firewall router box that I'm trying to write a ruleset for that
accepts/blocks traffic from Network A to Network B.  I'm testing the
rules on 3 virtual machines and will eventually deploy to production
hardware:

Network A Machine Eth0 <-------> Eth0 Firewall/Router Eth1 <------->
Eth0Network B Machine
              192.168.99.1                192.168.99.2           
              10.10.10.1       10.10.10.2


I have the the following rules on the Firewall/Router as a test before I
write rules with http, ssh etc:

iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -i eth0 -o eth1 -p icmp --icmp-type echo-request -s
192.168.99.0/24 -d 10.10.10.0/24 -m state --state NEW -j ACCEPT
iptables -A FORWARD -p icmp --icmp-type echo-request -m state --state
NEW -j LOG --log-prefix "ICMP: "

When I ping from 192.168.99.1 to 10.10.10.2 it does not work.  The log
rule logs the packet as IN=ETH1 OUT=ETH1.  I may not understand how the
interfaces should be referenced in the FORWARD chain, but I would think
that the second rule above should allow and forward that icmp traffic.  

However, if I remove the -i eth0 and -o eth1 from the second rule above
the ping works fine, the log of course still says  IN=ETH1 OUT=ETH1.  

I guess I don't have to reference the interfaces in all my FORWARD
rules, but I'd like to.  I am confused why the -i and -o referenced in
the second rule does not allow and forward traffic. And you the log rule
log the packets as IN=ETH1 and OUT=ETH1, I would expect IN=ETH0
OUT=ETH1.