My server does not forward packets between interfaces. What am I missing?

view story

http://askubuntu.com – I'd like to get my Ubuntu 11.04 server to route packet between its two network interfaces. The two ethernet interfaces (eth0, eth1) work apparently well as such. Both have static IP addresses ( & and both are reachable from their respective network. All IP services as well as the Samba server are reachable from both sides. sysctl -p returns "net.ipv4.ip_forward=1" and I can even ping the interface "on the other side" from both networks. In other words: I can ping from all PCs on the network and from all systems on the 192.1 ()