I have created a simple package to show a strange behaviour when parsing from URDF to SDF with Gazebo6 or Gazebo7 and ROS Jade (no problem with Gazebo5).
This is the example with a full description: https://bitbucket.org/alextoind/a_buggy_robot.
It seems that the problem affects only those URDF elements under
<gazebo> tags which would end under the
<collision> SDF elements.