This is not specific to VoIP, but it is nonetheless really cool. The Technology Review reports that a zero-emissions city is being built in Abu Dhabi, partially just to see if it can be done.
"The city--which will include apartments and laboratories, but also factories, movie theaters, cafés, schools, fire stations, and so on--is intended to generate as much electricity as it uses. Its water will be recycled to save the energy costs of desalination. Vacuum tubes under the city will transport garbage to a central location, where it will be sorted, and as much as possible will be recycled. Trash that can't be recycled will be converted to energy through a gasification process and the leftovers incorporated into building materials. Sewage will be treated and some of it processed into a dry renewable fuel for generating electricity. The transportation system will include a light-rail line linking the development to downtown Abu Dhabi and the airport, as well as a personal rapid-transit (PRT) system with stations throughout the city. The PRT, a system of automated electric vehicles, will connect people to the rail line or deliver them to parking garages outside the city."
One wonders what their phone systems will be. Since VoIP doesn't require separate wiring from the data network, there's already a green advantage there. It can even be argued that it reduces travel (and therefore carbon emissions), since your phone number can follow you and video conferencing is much simpler. The flexibility and capacity of VoIP PBXes compared to traditional PBXes make the answer seem obvious to us...but what will the designers of Masdar City decide? We can't wait to find out.