Thursday's NASA news conference is archived here:

http://mars.systemsfirst.com/news_archive/index.html

About 43 minutes in is discussed the dark deposits inside the crater. Dr. John Grant said the dark deposits in the crater were like a mystery material on the Humphrey rock. This material was either glass-like or a weathering product, presumably due to liquid water, since it was found to have been exposed to small amounts of liquid water weathering.
He said the dark material in the drift outside the crater was not like this, but more like the classic basaltic material.
If this mystery material is due to liquid water weathering, examining up close would give a chance to see a large-scale deposit of liquid water altered minerals at the Gusev site.

Bonneville in Color.
http://marsrovers.jpl.nasa.gov/gallery/press/spirit/20040315a.html

The dark mystery material is in the far side crater wall.

Bob Clark