Python3 and gcc46 isn't supported in ros yet, might be best to wait for a update. Originally from the ROS wiki. ROS is an open-source, meta-operating system for your robot. It provides the services you would expect from an operating system, including hardware abstraction, low-level device control, implementation of commonly-used functionality, message-passing between processes, and package management.
Get dependencies, and create alternate python dir to workaround shebang line in scripts that point to python3. Thanks to DavidS. Symlink libboost_filesystem.so to libboost_filesystem-mt.so, not an ideal solution, but it works. Then install rosinstall, and ros.
pacman -S python2 cmake wget python-yaml svn python2-distribute boost python-yaml pil python-numpy yaourt -S log4cxx mkdir /usr/local/bin/alt ln -s /usr/bin/python2 /usr/local/bin/alt/python export PATH=/usr/local/bin/alt:$PATH ln -s /usr/lib/libboost_filesystem.so /usr/lib/libboost_filesystem-mt.so sudo easy_install-2.7 -U rosinstall rosinstall ~/ros "http://packages.ros.org/cgi-bin/gen_rosinstall.py?rosdistro=diamondback&variant=desktop-full&overlay=no"
Follow the ROS wiki afterwards.
Tf package fails to build. You can apply this patch.