Here is some info on planned narrow body fleet changes after the merger. Some of it is good news most not so good.
The DC-9's will continue to be retired and the category shut down sometime in 2012. There will be 62 airframes left on 1 Jan. No airframes will come back to the line once retired. CASM very high.
MD-90 is still a player if Delta can locate at least 34 airframes. If not and the fleet can't be brought up to around 50 aircraft then the current 16 MD-90's will be retired. If they find enough MD-90 airframes and make the purchase some MD-88's will be retired so it will not be a net gain of 34 airframes.
737-700 fleet will increase to about 20 airframes. No plans beyond that. Aircraft purchased for specific airports and long thin routes. Any additional 737's will be 800's.
757's parked this fall at Delta will be brought back with modified FMC's (goodbye 200k boxes) in the Spring if bookings continue to hold up. Did not mention the parked NWA 757's however may have meant all of them also.
Here is the real bad news. There are no plans for a 100 seat aircraft. Looking at the Bombardiar C series however current plan is to use 76 seat regional jets in that market. Watch for the company to try and trade scope relief for contract changes to allow more in the 2011 time frame. (We should not even think about it!)
737-800's will see more deliveries but limited numbers. Number to arrive will depend on the timing of the next generation airframes from Boeing and Airbus. If those programs get pushed back again look for more 800's. If not then Delta would like to make due with the current fleet and MD-90's until that aircraft comes available in 2018.
Overall not a lot of fleet growth on the narrow body side and we may see a net loss of a small number of airframes depending on the MD-90's. If bookings hold up could be a small increase but nothing huge.