Comment 12 for bug 488340

I was able to get my issues resolved by using the proprietary drivers available on Broadcom's Linux site, and compiling the driver myself (replacing the one provided by the Ubuntu package): http://www.broadcom.com/support/802.11/linux_sta.php

The latest driver (5.60.48.36 as of this writing) worked flawlessly, and the random disconnect issues have been fixed. Ubuntu should be able to just upgrade their package to Broadcom's latest driver and resolve this bug.