In the design highlights, it has a diagram of the materials used in each section of the plane. The pointy part at the back is the only part made of titanium in the whole plane, does anyone have any idea why that would be?
This site [2] has a very good discussion about the Boeing 737 APU and APU exhaust (which they call an "Educator").
[1] http://en.wikipedia.org/wiki/Auxiliary_power_unit [2] http://www.b737.org.uk/apu.htm