To get the code:
(This will create a directory xf86-video-ati in your current directory and place the code there.)
git clone git://anongit.freedesktop.org/git/xorg/driver/xf86-video-ati
You will probably need a get a few -dev packages for your distribution for the build to go through. I can't tell you which ones though (I'm running Gentoo).
I also believe there are some recent changes to mesa - perhaps someone else can fill in the details?