I'm having this debian package on Natty:
ii ros-electric-ros 1.6.2-s1313696066~natty ROS core
with this file in it:
ros-electric-ros: /opt/ros/electric/ros/bin/rosversion
which reports:
$ /opt/ros/electric/ros/bin/rosversion -d
unstable
I recall that some time ago this worked and that it reported 'electric'.
Peter