The iptables package is on Maemo extras devel. There is no support for connection state on the device Kernel consequently the NAT is not working. I tried to compile the modules, but I found myself in trouble trying to load them at the device. If you want to flash a kernel with support for connection state there is one available at my personal repository (read: mWall :: netfilter + ui for maemo for more information). Another discussion about that modules can be found: here.