You haven't specified the nat table.
By default, if no table is specified, iptables assumes the filter table, and there is no PRE/POSTROUTING chains in the filter table.
So it should look like:
Code:
$IPT -t nat -A PREROUTING -i eth1 -p tcp ! -d 192.168.0.0/24 --dport 80 -j DNAT --to-destination 192.168.0.1:3128