I've been working commercially with netflow for some years now and have implemented systems which cope with billions of flows per day, calculating hundreds of customers data plans. While those systems run from cisco routers, this can be done under FreeBSD fairly easily and can monitor all the interfaces individually and then filter/aggregate them based on subnets, IP's, ports etc. The only failing currently with flow tools is no IPv6 support. I have built netflow v9 gernerator/capture software (which supports IPv6) in php to gain an understanding of it. Based on all of this knowledge, I am sure I can contribute something to the pfSense project, if so desired.