This was it... We took the quad to an open space and finally did a real flight test. No strings, no holding it down, nothing... And it worked!!!!
Here's the complete configuration of this working version (v1.0):
Arduino MEGA board
Y83 IMU 9-DOF (or similar)
4x Turnigy 2830-800Kv motors
4x ESC Hobbyking SS series HK-40A
4x Props 10x4.5
Turnigy 3600 mAh LiPO battery
Futaba T14SG 2.4GHz radio
Firmware: MultiWii v2.0