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.
Disadvantages: you don't get to visit the luxurious resorts at which literature conferences are so often held : ) And it may take you a little time to get up to speed with your audiences (no face to face chats in bars, etc.)—I found this highly workable, actually.