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
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