How would you dampen this little quadcopter?
Awesome little quadcopter there.  A few things to think about before using the more interesting filters:


Check that the desired acceleration signal isn't swamped by power supply noise or vibration.  This will show up easily in a high rate capture of raw values.  Try to get a capture with the motors on, off, and at low PWM where the noise can be particularly bad.  Even better would be pushing the values out a DAC to a scope while ramping the motor up, but work with what you have.
Watch out for power supply noise!  This has caused me a great deal of frustration, MEMS devices use switched capacitor techniques and can be very sensitive to supply noise that lands on the internal clock or a harmonic thereof.  Excerpt from ADXL323 datasheet below.