The BCM43xx driver from DragonFlyBSD has been ported to FreeBSD. Andrew Thomas and others have done the port. I've pushed it into the tree, and plan on fixing the one or two quirks with my card.
The driver is far from perfect. 802.11a isn't support. Some older BCM4306 cards don't support channels 1, 2 or 3. It is rumored not to work at all for some people.
The driver was written by Sepherosa Ziehau for DragonFlyBSD. He used the specs available at http://bcm-specs.sipsolutions.net/Specification to write the driver. The driver is written against the v3 firmware. The v4 firmware is needed for support of newer cards, and many people have made noise about porting to that, but nobody has emerged yet to do that work.
The driver will be in FreeBSD 8.0. A back port of the driver to 7.x is difficult because it relies on all the new VAP work that Sam Leffler has done that is not in 7.x (although backports exist for 7.x, they won't be committed to the FreeBSD tree because it breaks binary compatibility).
You need the net/bwi-firmware-kmod port installed for the firmware. Although the binary .o file that the firmware is extracted from is apparently easy to distribute, distributing derived works of that .o file is not. So, there's a port to download, extract the firmware and make a kld for the driver to access.
A man page will be committed soon.