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This sounds like the rostopic echo node isn't receiving the callback from the ROS master that tells it that there's a new publisher, which is probably why it isn't subscribing or seeing any messages.
There are a number of things you can do to debug this.
With just the roscore and the rostopic echo node running, inspect the rostopic echo node. First, find the node name with:
rosnode list
And look for a node name that starts with /rostopic
. It will probably be something like /rostopic_36138_1400087540700
. The inspect the node by running:
rosnode info /rostopic_36138_1400087540700
If it can't contact the node for some reason, troubleshoot your network configuration and your /etc/hosts file to make sure that the name advertised by the node resolves, either through DNS or through your /etc/hosts file.
If you're running the master on a separate machine from your Pi, also try running rosnode info
from there, to make sure that the ros master can successfully contact the node to tell it about new publishers.