That is exactly the problem KLM has. They operate both 737-800 aircraft as the stretched -900 series. Problem is that they both accomodate 180 passengers against higher operating costs for the -900. Indeed it has something to do with the overwing escape exits.
I heard Alaska airlines will exchange two Boeing 737-900 it has on order, with two 737-800 aircraft. I guess it doesn't have anything to do with the seating capacity though.
Marcel_MPH
