ROS C Turtle Alpha 6 has been released, which includes ROS 1.1.15. We
are getting close to our release candidate for the actual C Turtle
release. As we approach this release candidate, we anticipate more
rapid releases to quickly verify final code changes going into the
system.

More details here:
http://www.ros.org/news/2010/07/ros-c-turtle-alpha-6-released.html