Not enough to be able to build a complete bot running ROS on an Arduino, but enough to make it easy to use Arduino as part of a bigger system using ROS. A much more complete implementation will run on Andriod [in Java].
If the power needed to drive the transmitter leds (and total amount of light) is in a reasonable range, this technique could be used to communicate with or between [mobile] robots. This looks like much the same concept as infra-red remote controls, but with considerably higher bandwidth. The visible light is more line of sight though. Any opaque object in the light path would block the signal. Some infra-red systems will ‘bounce’ signals off of surfaces to work ‘around’ direct obstructions.
For robot communications [mesh network], maybe more directional channels would work better [to reduce power requirements]. Use one or more leds in a curved reflector. Depending on the shape of the reflector used, that could limit the communications to a single ‘target’, or maybe only to [near] a single ‘plane’.