It seems they are able to bounce light off these tubes, and based on the color of the reflected light can determine the state of the tube's charge. The goal is to see how feasible these creations are for use in carrying current, and ultimately be a part of a greater whole such as even smaller computers:
In order to make carbon nanotubes useful in building logic circuitry, scientists are pushing to demonstrate their high speed, high packing density and low power consumption capabilities as well as the ability to make them viable for potential mass production.
I won't pretend to understand it all, but it is very fascinating stuff. It was a little over a year ago that IBM first built an IC using carbon nanotubes. Over the more than 100 years they have existed, IBM has played a massive part in the development of computing technologies. Obviously that continues to be true. You can read the press release at IBM's site.