According to the type changes from 9.3 to 10 the four Coclasses that implemented IMetadataImport interface have been removed. These are ImportXML, ImportMP, ImportMPTXT, ImportMPSGML.
Now the IMetadataImport interface documentation just says:
The IMetadataImport interface is available to custom CoClasses that implement the MetadataImport abstract class
Custom CoClasses? that's it?
I have written a program to help facilitate the creation and maintenance of metadata and part of it relies on automating the import metadata templates as xml (in the ISO 19139 spec). ArcCatalog 10 still supports importing xml metadata, so where are the ArcObjects to complete this task?