note that "the top of the slip should be on the left side of the way, and the bottom on the right side".

some variation on 'natural=cliff'?
--
RP: these are all i can find. they're not great, and are unlikely to be used/accepted elsewhere
http://wiki.openstreetmap.org/wiki/Proposed_features/Terrain_accident
http://wiki.openstreetmap.org/wiki/Humanitarian_OSM_Tags/Humanitarian_Data_Model#Obstacle
maybe we make up natural=landslide?
--
HB Aug 2012: I think making up natural=landslide is a reasonable solution.

see also http://wiki.openstreetmap.org/wiki/Tag:hazard_type%3Dlandslide