A Cryovolcano?

Alright, so my bro (jackelope) and I were looking at some of the Casinni Huygens stuff and they started talking about the Cryovolcano that they might have found and I, being the scientific infant that I am, need someone to spoon feed an explanation to me. So how does a cryovolcano work exactly? Or how does it work in theory?

I realize that it exists in a different atmosphere and that temperatures and such are much different, but I still can’t grasp what mechanic would drive a mountain to spit ice instead of lava…

So Straight Dope, please hear my cry!

Wikipedia has a decent little artice on Cryovulcanism
For more information than you probably want, see Cryovolcanism on icy satellites: Application to Titan.
For an earthly example of near cryovolcanism (only 500 to 590°C, 930 to 1090°F) see Ol Doinyo Lengai in Tanzania. There are some lovely night time pix of that volcano here.