Hm, the IPNAT howto you referred to doesn't look very complete.  I
recommend the one linked off of www.netbsd.org (under howtos ->
networking), although I didn't find a few problems with it.  Most
importantly, make sure that your kernel is compiled with options
GATEWAY or that you run "sysctl -w net.inet.ip.forwarding=1" in
/etc/rc.local.