I'm not sure how many of you have heard about this project, however I found FreeBSD ringmap implementation when I was googling and it seems to be interesting to me, I suggest you visit the link and read up the documentation/presentation.
I'm going to try it out whenever possible, right now it is ported to FreeBSD 8.1 stable, you can actually download the source code and test it out yourself.
You can also find a lot of information about high performance packet capture from the link below as well, I usually use the setting that is recommended over there for my FreeBSD sensor setup.
By the way, FreeBSD already has zero copy bpf implemented, thanks to Robert Watson for that since he has done a lot of background works on it. To know more about zero copy bpf you can check the presentation slide here -