Some officials at Boeing (the source here is a Seattle paper), say a more near-term and industrywide approach is what's required. Mike Cave, vice president of business strategy for Boeing Commercial Airplanes, told the Seattle Times that, "Fifty years is a long time. Do we have a vision today for what technology will bring us to a zero-emissions airplane? We don't."
Giovanni Bisignani, chief executive of the IATA, on the other hand, called the zero-emissions aircraft "absolutely achievable."
Cave suggest the airline industry should instead work towards being carbon neutral in the near future (using more efficient engines, better taxiing procedures and a global carbon-emissions trading system, for example).
There's lots more details in the Times article, and here are some more thoughts on greener flying: