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