Installed 12.04 on HP laptop with wireless hardware switch. Apparently no driver included in 12.04, and can't find one via driver update. Worked fine with 10.04 and Win XP. Seems to me that Broadcom b43 is what worked with 10.04 but I'm not absolutely sure about that. Ideas?
Try shutting down(if not already off), disconnecting the AC power, remove battery, press power a few times to release stored energy, reinsert battery, reconnect AC power. Turn on Laptop. Worked for me. After trying multiple fixes I found on the internet I remembered reading this somewhere. My hardware was blocked via a hardware switch(fn+f8) which doesn't work in ubuntu(works with windows, but uninstalled windows). rfkill showed 0: phy0: Wireless Lan as Hard Blocked.
Have you worked way through this? This has worked for me in the past https://help.ubuntu.com/12.04/ubuntu-help/net-wireless-troubleshooting.html
Broadcom wireless cards are a little complicated with linux. Like nVidia and Radeon, Broadcom provides linux drivers, but they are not open source. Ubuntu has Jockey (or Additional Drivers Tool) to help with this. Apparently, Jockey has a hard time identifying certain broadcom chipsets  and loads the wrong driver. This is also covered in the Ubuntu Documentation.
I have had the best luck installing the b43-fwcutter and firmware-b43-installer packages, but this may be different for your hardware.
 - Ubuntu Forums Topic #11923344 post #2. The whole topic may be helpful to you as well.
Are you absolutely sure it is the wireless driver which is not working? I found that one router was set to channel 13 and some wireless card drivers won't work above Channel 11. Something to do with US Channels vs rest of the world.
If possible check the wireless router is at a channel between 1 to 11 and re-boot. (This is unlikely to be the cause if you ARE in the United States).