Hey, I don't know if it's been done before, but when I last looked I couldn't find much. Anyway, I've taken Peter Stamfest's iptables-snmp net-snmp library, fixed it up (it had a few memory leaks and couldn't handle multiple oids in a single packet) and put it in a package.

http://www.nobiscuit.com/iptables-snmp/

The snmpd/libnetsnmp packages that come with whiterussian aren't compiled with the options that allow loadable libraries to work, so I've included those packages as well. The source is there if you want to have a look. The code is pretty ugly, but it works for me: http://www.nobiscuit.com/mrtg/

All the graphs except for the top two are generated from polling iptables rules on my wrt54gs.