I recently ran into some problems when I created an OpenBSD based bridge fw using fwbuilder. I used a vanilla OpenBSD 5.4 install and I created all of the nics and bridge interfaces in fwbuilder.
However the install script generated by fwbuilder uses the addm och deletem commands for setting upp the bridge. I hade to change these to just "add" and "delete" in order to get everything to work.
I changed this in the "/Applications/fwbuilder-126.96.36.19999.app/Contents/Resources/configlets/bsd/update_bridge" file and everything worked fine. (I use the Mac and linux version of fwbuilder)