Post made by me in April.
The 737/A320 replacement is still about 10 years from entering service. There are several reasons for this. First, the 737/A320 were by far the best selling commercial aircraft of all time. So it is crucial for Boeing and Airbus to get it right. Not only will the 737/A320 replacement have to cover the midsized 737 sized market (about 150 seats), but it will need to cover both the 757/A321/737-900 on the upper end (around 200 seats) and cover the EMB-190, 737-500, A318, DC-9, 717 on the lower end (around 100 seats).
An obvious feature which you will see on both of these aircraft is a lightweight composite structure. Since Boeing is doing this for the first time (on a commercial aircraft) for the 787, if would be in their interest to let the 787 fly for 2-3 years to see what, if any bugs need to be worked out. The other improvement is going to have to be the engines! Weight and engine efficiency are by far the two most important parameters that affect an airplanes efficiency. The field of aerodynamics is already very advanced, and the only significant improvement that can really be made is to use a flying wing body. So while there has been a breakthrough in weight, there has not been a major breakthrough in engine technology. A major advance in engine efficiency is going to be the driver for the launch of the 737/A320 replacement aircraft. Boeing is envisioning a 2015 timeline when this new technology can become available.
This new 737/A320 replacement aircraft is very important for both Airbus and Boeing. Also, expect Bombardier and Embraer to creep in on the lower end of the market. This will be a hard faught competition for this new generation aircraft. This is good news for the airlines, but they need to be patient. Expect about 10 years before they start flying scheduled service.