Developers associated with the community-spawned b43 driver have begun programming the final component needed to having a working open-source wireless stack on Broadcom hardware: a working firmware. These developers have written a simple 802.11 compliant firmware, but it's not yet feature-complete.
This open-source firmware is compatible with the mainline b43 Linux driver and doesn't require any modifications to the driver since the global registers and shared memory are being treated in the same way. This firmware should work with Broadcom BCM43xx 802.11b/g WiFi adapters.
The announcement regarding the open-source Broadcom WiFi firmware can be read on the bcm43xx-dev mailing list. The community-developed firmware can be found on the OpenFWWF web-site (Open FirmWare for WiFi networks).