I could clearly see that something was coming out from the fuel dump (jettison) valve:
I had taken this picture from my seat before the flight.
The incident took only a few seconds - the valve was only dripping a little bit when I had my camera out. But, it definitely looked like in this picture:
On a side note, have a look at an Airbus overhead panel:http://www.avsim.com/pages/0108/Airbus2/pic_05_12.jpg
The fuel jettison buttons are placed the middle of the left panel.
Now compare to this (supposed to be from a 767-300):http://contrailscience.com/skitch/Overh ... 103650.jpg
Given Airbus' rather inconspicuous fuel jettison buttons, could it be the pilot hitting these buttons by accident?
What happens in the cockpit when you do that, is there a warning?