Well, the closest experience I've had to what you ask is when I once decided to pump water from a river towards a lava pool, to see if it would seal the creatures inside. The fire imps had been climbing out of the vent and wandering near my fortress, making work treacherous for any dwarves who needed to go outside.
At first, the exposed surface of lava began to turn to obsidian, as expected. However, being the curious type who likes to experiment, I had instructed some miners to break into the base of the lava pool from underneath (this lava actually had a bottom; it was not connected to a bottomless magma flow), which allowed the lava to leak into my huge basement, draining the pool.
The water was still pumping into the lava pool. It was spreading across the layer of obsidian that had formed across half of the pool's surface, and would then drip down onto the now half-empty pool of lava. Unfortunately, since the water was landing in the middle of a pool of lava, there was nothing to support the resulting obsidian, and constant "cave-ins" resulted, as each block of obsidian that was formed immediately crashed down to the bottom z-level.
I had a constant flood of messages telling me that a section of the cavern had collapsed. This went on for a long time, and the fact that I had to unpause it every time a drip of water fell into the lava shaft meant that I soon tired of it, and quit to reload my last save.
It was still fun to try, though.
[ December 14, 2007: Message edited by: Tayrin ]