Hopefully this helps someone to get started.
First I followed gentoo xorg and ati related documentation
run aticonfig --initial (see --help for details)
Then I had to 'emerge amd-app-sdk-bin', you get the e-build from https://bugs.gentoo.org/304903?id=304903
You also need the app-admin/eselect-opencl (http://trac.pentoo.ch/browser/portage/trunk/app-admin/eselect-opencl?rev=1839&order=name
You probably need the pyopencl for the miner software so run 'emerge pyopencl', it is going to pull in the dev-util/nvidia-cuda-toolkit. Do nor worry about this! You can run: 'eselect opengl list' and select ati
Have fun and happy mining... in my pool obviously