It’s pretty easy to install the broadcom firmware and get the card running. I put my ATV on a wired connection and SSHed into it (but you could probably use the local terminal or a spare wifi dongle). Then just use the following commands:
apt-get update apt-get install firmware-b43-installer
(Maybe you need to do sudo su first, can’t remember)
Worked for me, now I’m using the internal wifi card. Only weird thing is: I can’t connect to any n-only wifis, had to setup my router to b/g/n for the card to connect.