Thanks Josh,

It works.

From: Josh Faust <jfaust@willowgarage.com>
To: User discussions <ros-users@code.ros.org>
Sent: Thursday, October 7, 2010 5:32:06
Subject: Re: [ros-users] Communicating between two user created packages

undefined reference to `my_namespace::X::X()`

You need to add the linker flags to the exports as well. For example, from roslib:

lflags="-Wl,-rpath,${prefix}/lib -L${prefix}/lib -lroslib"

Josh