The California Geologic Survey has published a new map of landslide susceptibility for the entire state of California. It addresses only deep-seated landslides, not shallow slides such as debris flows. The criteria used for developing the map included slope angle, geology, rock shear strength and history of landslides in a given area. You can download the map for free from the California Geologic Survey.
Folks in Oregon are angry about a recent landslide and debris flow that closed Highway 30 and destroyed property in the town of Woodson. (Here is a powerpoint presentation by Bill Burns of the Oregon Department of Geology and Mineral Industries showing how the event happened). Some rather stinging allegations are being made that politicians withdrew a landslide hazard map from general use in 2002 for political reasons. That map or another one had characterized the area effected by the December 2007 debris flow as a "very high" or "extreme" risk of landslides. A very interesting article.
The Utah Geologic Survey has released a "Landslide Susceptibility Map of Utah". They apparently relied quite heavily on GIS based thresholding of existing slope angles but only after they had statistically analyzed failure angles for particular geologic units. So it sounds like they throw the known landslides, the geologic map of Utah and a DEM into the GIS a blend it all up. Perhaps a slight oversimplification!