In the documentation for AttacheInterrupt,
the example doesn't show that PD2 (INT0) is being configured as an input pin.
Do the external interrupt pins need to be configured for INPUT when using them as interrupts only?
Pins not explicitly configured as outputs are inputs by default. So no, it should not be necessary to configure these pins as inputs (though it doesn't hurt).
The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons
Can also use the internal pullups on them.