Update Nov 11, 2009 :
Do not follow this post and please update your installed package using your favorite package manager such Synaptic or apt-get or whatever and your Broadcom BCM4312 will working properly :

sudo apt-get update
sudo apt-get upgrade
sudo apt-get install linux-backports-modules-karmic

If your Broadcom wireless BCM4312 did not working after upgrading your packages because of you [...]