USB Networking troubles

[arne anka]

debian/unstable amd64, 2.6.26.
my /etc/network/interfaces says for usb networking

allow-hotplug usb0
iface usb0 inet static
         address 192.168.0.200
         netmask 255.255.255.0
         network 192.168.0.0
         post-up iptables -A POSTROUTING -t nat -j MASQUERADE -s  
192.168.0.0/24
         post-up echo 1 > /proc/sys/net/ipv4/ip_forward
         post-up iptables -P FORWARD ACCEPT

you should try the last three lines in a shell, to see if helpful messages  
come up (i had to build an additional iptables module).