2.0 supports interface grouping via lagg. http://www.freebsd.org/cgi/man.cgi?query=lagg&apropos=0&sektion=0&manpath=FreeBSD+7.2-RELEASE&format=html
I would suggest grabbing an Alpha snapshot if you want to experiment with it.
The underlying lagg features should be available in the 1.2.3 snaps, so you might be able to hack something together using the shell, scripts, or something. I haven't tried that, and it would be unsupported, but it might be an option if the 2.0 snaps seem too volatile for your needs.