People often listen to music when they run to enjoy the relaxing effect it can have. As an alternative, they may also enjoy the social experience when running with someone else. Unfortunately, going for a run with someone else without music on often amounts to you running behind (or in front) of them just listening to yourself breath, which can really diminish the quality or effectiveness of the run.
Consider a small broadcasting adapter for an iPod - essentially the existing iTrip works as a good model. One person would attach that their player while another person wears headphones hooked to a tiny receiver (instead of a music player). The result would be a shared listening experience that could enhance the social exercising experience. Clearly, this could be used in a range of situations, and for that matter, may already exist, but I like the concept nonetheless.