3.0 it is
metaDS.RetrieveMetadata(MetadataFlags.EntitiesOnly)
where metaDS is an instance of the metadata service.
4.0
CRM4.MetaDataSDK.RetrieveAllEntitiesRequest rer;
rer.MetadataItems = MetadataItems.EntitiesOnly
RetrieveAllEntitiesResponse metadata = (RetrieveAllEntitiesResponse)service.Execute(rer);
*Folks at MS got bored with doing things faster in 3.0 ...
Cheers,
Karlo