Dr. Christopher P. McKay gives a highly technical but visually arresting presentation to Google about the geology of Mars, based on the discoveries made by the Phoenix lander:
I started reading Blue Mars recently, the third and final part of the trilogy by Kim Stanley Robinson, that focuses heavily on many of the themes discussed in this presentation. Well worth reading if you have any interest in the terra forming of Mars.
Updated 2021 : note that the above post is from 2008, but is left here for archival purposes. I have updated it to embed the correct video directly.