GoussLegend:
Hum I think I finally understand. Some pull-up resistors should do the trick: http://playground.arduino.cc/Main/I2CBi-directionalLevelShifter
Read the page carefully. It stays you have to know the risks and suggests that making a 5v arduino recognise a 3.3v signal it's living on the edge.
Quadcopter is a very complex thing and if something fails there is a risk of crashing or losing control or some thing else. Think of this, what if the sensor fails while it's flying. So, it's better to buy an IMU that supports 5v or better yet but a MultiWii.