I've installed 12.04 on my mac pro 9,1 and the wireless has been a real pain.
I installed the b43 (linux-firmware-nonfree and b43-fwcutter).

Sometimes, I have to manually kick the wifi scan into action by using

sudo iwlist wlan0 scan. Otherwise in the network manager, it simply shows "wireless disconnected" anfd doesn't scan.
At this stage, dmesg shows that wlan0: link is not ready. After the manual scan, somehow, network manager picks it
up and displays SSIDs and from there, a connect works fine.

But then, after connection, it gets very unstable and it keeps dropping.
dmesg at this stage shows deauthentications (I've seen reason 2 and 6 the most often).
In the settings for a wireless connection, I disabled ipv6 and set the MTU to 1500. Unfortunately still instable.

kernel version: Linux version 3.8.0-29-generic (buildd@panlong) (gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5) ) #42~precise1-Ubuntu SMP Wed Aug 14 16:19:23 UTC 2013

lshw -class network
description: Wireless interface
physical id: 2
logical name: wlan0
serial: a8:bb:cf:21:f2:6a
capabilities: ethernet physical wireless
configuration: broadcast=yes driver=b43 driverversion=3.8.0-29-generic firmware=666.2 ip= link=yes multicast=yes wireless=IEEE 802.11bg

Please Help!