smbd never ask winbind for a idmapping on sid_to_uid() if the sid belongs to 
the local machines SID.
If someone wants using a central idmap database this is a bug!

sid_to_gid() works correct...


this is in sid_to_uid():
...
	/* if this is our SID then go straight to a local lookup */
	
	if ( sid_compare_domain(get_global_sam_sid(), psid) == 0 ) {
		DEBUG(10,("sid_to_uid: my domain (%s) - trying local.\n",
			sid_string_static(psid) ));
		
		if ( local_sid_to_uid(puid, psid, &name_type) )
			goto success;
			
		DEBUG(10,("sid_to_uid: local lookup failed\n"));
		
		return NT_STATUS_UNSUCCESSFUL;
	}
	
	/* If it is not our local domain, only hope is winbindd */

	if ( !winbind_lookup_sid(psid, dom_name, name, &name_type) ) {
		DEBUG(10,("sid_to_uid: winbind lookup for non-local sid %s 
failed\n",
			sid_string_static(psid) ));
			
		return NT_STATUS_UNSUCCESSFUL;
	}
...

this is in sid_to_gid():
...
	if (!winbind_lookup_sid(psid, dom_name, name, &name_type)) {
		DEBUG(10,("sid_to_gid: winbind lookup for sid %s failed - 
trying local.\n",
			sid_to_string(sid_str, psid) ));

		if ( local_sid_to_gid(pgid, psid, &name_type) )
			goto success;
			
		DEBUG(10,("sid_to_gid: no one knows this SID\n"));
		
		return NT_STATUS_UNSUCCESSFUL;
	}
...