roslaunch'ing does not seem to be working with Gazebo 8.
I'm not sure if this is a ROS, Gazebo, or ros_gazebo issue, but I thought I'd see if anyone here has a solution or a good method to trouble shoot.
Any roslaunch of the gazebo_ros package will open Gazebo, and seemingly start up ROS packages, but it won't actually launch the world file. To replicate, install Gazebo as described above and roslaunch gazebo_ros mud_world.launch. You'll see none of the models in this launch/world file show up.