It seems that Boeing are playing the long game in revealing their latest launch decision:http://www.flightglobal.com/articles/2007/06/25/215025/boeing-expects-to-deliver-737-replacement-by-2015.htmlI guess technology will change quite considerably by then but trying to take 15% off operating costs (compared with the 737) will take some doing, especially if it take into account environmental concerns.
Like Bombardier with the C Series, Boeing won't want to launch the 737 replacement until they know that it can offer that kind of reduction in operating costs, which will be primarily linked to the development of more efficient engines. Boeing also knows that Airbus is going to be very busy with the A350 XWB and A380 and is bound to follow them into the market with their A320 replacement. Pratt and Whitney want to proceed with their geared turbofan, but maybe the propfan will still have it's day.
maybe carriers like SAS or/and American Airlines are finally waiting for such new designs to replace large portions of their fleets from ca. 2015 onwards. At least SAS wants to wait and announced in their latest annual report that their remaining MD-80-fleet will probably be retained until such new aircraft are coming online. SAS does not see enough economical advantages by replacing their current MD-80-fleet by additionial "currently produced" aircraft and decided to look for ways to reduce the noise-impact of their MD´s until the replacement is found.
The Winner Takes It All