Question: Can you run a [BLDC motor](http://en.wikipedia.org/wiki/Brushless_DC_electric_motor) backwards without damage?

Is it OK to drive a model airplane BLDC engine backwards while landing, so it gets a little "reverse thrust" and come to a stop a little quicker on the runway?

Is it OK to drive a model helicopter BLDC motors backwards so it can hover upside-down?

Or do I need to design the hardware so that it never drives the motor backwards, under any circumstances, no matter what the pilot on the ground does at the transmitter?

Answer: 
Thrust on helycopters is controlled by varying the propeller pitch, not the motor speed/direction. In an helycopter, the main rotor motor turns almost always at the same speed. 

Inverted hovering needs a special designed [swash plate](http://en.wikipedia.org/wiki/Swashplate_%28helicopter%29) that allows positive and negative blade pitch. 

For the original question, yes, you can drive a brushless dc motor in both direction. 
Stopping it quickly (with a propeller connected to it), change direcion to the thrust, and keeping your plane on the track, is another story :-)
I know this is a very old question from 2010, but for the sake of other people with the problem today, brushless drone and model motor controllers now commutate the motor by sensing the back emf of the winding. There is no hall effect sensors, so you just have to reverse any 2 of the 3 motor wires.