Hi,
You can find the patch here
http://people.freebsd.org/~thompsa/if_bridge-5stable.diff
1) Apply the patch
# cd /usr; patch -p0 < if_bridge-5stable.diff
2) Add 'device if_bridge' to your kernel config (or use the module)
3) Do the "buildworld, buildkernel, installkernel,
installworld"-dance
Feel free to test and report any problems. configuration is the same as
in current, see if_bridge(4) for details.
cheers,
Andrew