The FreeBSD Handbook states that "if the bridge host needs an IP address, then the correct place to set this is on the bridge interface itself rather than one of the member interfaces."
With FreeBSD 6.x and FreeBSD 8.0-CURRENT, setting the IP address on the bridge works without any problem. With FreeBSD 7.0-RELEASE, however, this does not work. The OS doesn't even properly boot when the IP address is set via /etc/rc.conf.
This is probably a temporary solution, but try setting the IP address to one of the member interface. This is in direct opposition to the advice given in the Handbook, but it works for me.
If this is a bug (which I suspect it is), a patch probably exists somewhere. Please feel free to leave a comment.