iptables, bridging and inside-/outside-issue

After reading through the article series by Mick Bauer regarding transparent firewalls I got a bit inspired to try for myself.

I've installed ubuntu onto a machine with 3 network interfaces, and bridged these three interfaces to one common bridge.

I've copied the iptables-script from part V in the series, but re-written it due to the fact that I in my installation will be unable to sort traffic based on ip.

I will not know which addresses will be used on either side of the firewall, so I'll have to sort my traffic on some other variable.

I was thinking that I could sort the traffic based on PHYSIN and PHYSOUT in iptables, but -i and -o does not seem to do that.

As an example I've created the following rule:
iptables -A FORWARD -i eth1 -o eth0 -p tcp --dport 80 -j ACCEPT

But I still get the following in my kernel log:
floyd kernel: [269519.979985] Dropped by default (FORWARD): IN=br0 OUT=br0 PHYSIN=eth1 PHYSOUT=eth0 SRC= DST= LEN=60 TOS=0x00 PREC=0x00 TTL=64 ID=31771 DF PROTO=TCP SPT=35727 DPT=80 WINDOW=5840 RES=0x00 SYN URGP=0

Which as far as I can figure should be allowed by the rule.

What can I do to sort the traffic based on the physical interface?