RViz does not respect transparency (alpha) for a simple URDF component (e.g. cylinder)

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Hello,

I am trying to make a part of my URDF model semi transparent since the part is a clear 12" acrylic plate. So I tried making the plate a flat cylinder with a light grey color and alpha set to 0.1 as shown below. However, the plate looks solid grey in RViz--the same as if I had set alpha to 1.0. I can make the whole robot more or less transparent by changing the alpha property for the Robot Model display. However, I want just the plate to be transparent. Below is a test URDF file that illustrates the problem. I am trying this using the latest Electric debian packages under Ubuntu 10.04.

<?xml version="1.0"?>

<!-- XML namespaces -->
<robot xmlns:sensor="http://playerstage.sourceforge.net/gazebo/xmlschema/#sensor"
       xmlns:controller="http://playerstage.sourceforge.net/gazebo/xmlschema/#controller"
       xmlns:interface="http://playerstage.sourceforge.net/gazebo/xmlschema/#interface"
       xmlns:xacro="http://ros.org/wiki/xacro" 
       name="test_urdf">

        <link name="base_link">
            <visual>    
                <origin xyz="0 0 0" rpy="0 0 0" />
                <geometry>
                    <cylinder length="0.005" radius="0.15" />
                </geometry>
                <material name="clear_color">
                    <color rgba="0.5 0.5 0.5 0.1"/>
                </material>
            </visual>
        </link>
</robot>