Have you specified the protocol?

Fished from the intarweb:

"The --to-ports option specifies the destination port, or port range, to use. Without the --to-ports option, the destination port is never altered. This is specified, as above, --to-ports 8080 in case we only want to specify one port. If we would want to specify a port range, we would do it like --to-ports 8080-8090, which tells the REDIRECT target to redirect the packets to the ports 8080 through 8090. Note that this option is only available in rules specifying the TCP or UDP protocol with the --protocol matcher, since it wouldn't make any sense anywhere else."