737 Upgrade
I visited Boeing recently, and got a brief on the 737. Right now Boeing has got a backlog of units, if I remember correctly it is something like three years, and orders are still coming in. What I understood, is the 737 is what is saving Boeing. There were planes parked all over the place, and their output is impressive. The manufacturing has be amortized quite awhile ago. With that airframe, and its popularity, I would think that a re-engined 737 would be pretty competitive, particularly when you also consider the existing facilities and maintenance equipment. That's my opinion anyway.