According to an answer I got from answers.ros.org, the proper way to
determine the 'rosws merge' target is 'roslocate info --distro=fuerte
perception_pcl', but the information is bogus. The closest tag is an
unstable 1.2.3 version, but that doesn't build on my Ubuntu 12.04,
fuerte 1.8.9 from source system. When I do try to build the unstable
version, I get bizzare compile errors relating to 'ros::message_traits
missing value' and other template barf looking errors. Using the last
stable release, 1.0.3 is inappropriate for fuerte because it relies on
now deprecated stacks (Eigen in particular).

A little more context.. My goal is actually really simple to state:
Build the velodyne stack from source for fuerte.

Here's the compiler error that I get attempting to compile perception_pcl:

/opt/ros/fuerte/include/message_filters/sync_policies/approximate_time.h:552:5:
error: ‘value’ is not a member of
‘ros::message_traits::TimeStamp<pcl::PointIndices, void>’

My goal is actually really quite simple: Build the velodyne stack from
source for fuerte.