Having built a Centos server using eth0 as the wan and eth1 mapped to br1 as the lan for KVM the firewall prevents Samba from being recognised by other clients on br1. I've tried firestarter, shorewall and the inbuilt firewall within Centos - all have trouble with the bridging of eth1 to br1 in that they need to be turned off for it to work. Setting the rules for Samba has no effect. The virtualized guest is either Win 2003 server or Win XP pro configured for the correct workgroup netbios name. It seems iptables can't handle the complexity of the bridge. Anyone else had this problem or knows of a workaround?

brak44, how's the interfaces file on your shorewall?
I have my br0 like this and it works fine:
lan br0 detect tcpflags,nosmurfs,routefilter

You might have some of the default parameters for the br1 interface, like it was in eth0, that CAN'T be used with bridge.

Rebuilding the machine has fixed the problem. I can only put this down to the many differing howtos that were tried in getting KVM configured in bridge mode in the first place. Using the Package Manager to just install KVM (not Virtualization) was a start.