If you manage to compile the Broadcom wl driver from source, you may run in to the “unknown symbol in module” when you try to load the driver with insmod. modprobe may even complain that the module doesn’t exist (the nerve!).
The answer to all these problems is simple: you’ve forgotten a very important step – to depmod! To rectify this problem, simply run depmod as root, and viola! You can now modprobe wl with no problems, and the module will be loaded automatically at startup too!