Many commercial airliners cannot actually dump fuel in flight. That is why you will see stories of airliners circling an airport for hours before trying to land on bad landing gear or otherwise.
http://en.wikipedia.org/wiki/Fuel_dump_systems
from the above link:
"most aircraft of this type [twinjet] such as the Boeing 737 (all models), the DC-9/MD80 and Boeing 717, the A320 family, and various regional jet ("RJ") aircraft do not have fuel dump systems installed"
Best regards,
~firebup