Right now, I’m rocking XBMC 11.0 Eden Beta 1, and just recognized by accident that it has AirPlay built in. I tried it, it works flawlessly.
In the current version, it offers video/audio streaming or just the audio, so the problem the article linked to above mentioned has been worked out. Just go to ‘System/Network’ and ‘allow XBMC to receive AirPlay content’.

Just like that.

By the way: If you’re wondering how the heck to install XBMC on Ubuntu 11.10, here’s how: (It’s some user’s private PPA that offers builds for current Ubuntu versions, since the official PPA is abandoned somehow)

sudo apt-add-repository ppa:nathan-renniewaldock/xbmc-stable
sudo apt-get update
sudo apt-get install xbmc

XBMC’s also available for Windows and Mac OS X.