Click here for an account of a green videoconference I just did for ASLE UK (the Association for the Study of Literature and the Environment). Science has just done a piece about it.
It seems so obvious that in the future we will need to reconfigure conferencing so that their dates overlap! That way keynote speakers can be shared by videoconference without wasting carbon.
Polycom is a pretty neat, cheap application on the new pcs that supports excellent videoconferencing (better than Skype).
Advantages: saving money, carbon; getting 1+n lots of feedback for the price of one; no jet lag.