So I was being dumb and griefing my own map with lava, (this took place on an ocean area covered in a layer of ice) and I went underwater. There was still liquid lava flowing down. I went up and poured water over all the lava on the surface, and the lava underwater only started cooling when the source blocks cooled. If I remember correctly from using a more creative items mod in 0.6.1 and grieving tons of my own maps with it, ALL lava cooled on contact with water.