General crosswalks listed at: http://libraries.mit.edu.ezproxyberklee.flo.org/guides/subjects/metadata/mappings.html . Includes an FGDC to MARC crosswalk, but from 1997: http://www.alexandria.ucsb.edu/public-documents/metadata/fgdc2marc.html
DDI (version 2?) to Dublin Core Mapping: http://www.ddialliance.org/DDI/related/dc.html
Harvard-MIT Data Center Dataverse Network Software mapping of elements in their catalog to DDI 2.1 to Dublin Core to FGDC: http://thedata.org/files/dataverse/CatalogingFields11Apr08.pdf
ICPSR discussion on mapping of DDI to MARC (doesn't list the mapping, but I'm sure I could get it): http://www.icpsr.umich.edu.ezproxyberklee.flo.org/icpsrweb/ICPSR/or/metadata/marc/