Update: as pointed out in the comments below, instructions are available on the ROS wiki. [sic The comments are lost to time.]

I couldn’t find any instructions for this. So, after some poking around, here’s the quick version (on Ubuntu Lucid):

sudo apt-get install svn build-essential cmake libboost-all-dev liblog4cxx10-dev
svn co https://ros.svn.sourceforge.net/svnroot/ros/trunk ~/ros_src
export PATH=$PATH:~/ros_src/bin
export ROS_ROOT=~/ros_src
export PYTHONPATH=$ROS_ROOT/core/roslib/src:$PYTHONPATH
make
rosmake roscpp_tutorials

I had to fix a few missing includes in core/roscpp/include/ros/xmlrpc_manager.h and

core/roscpp/src/libros/param.cpp but YMMV.

If you haven’t heard of ROS, check it out!