This post describes a series of Python scripts to convert FGDC XML metadata for WMS and WFS services to ISO 19139 service metadata via GeoNetwork's OGC harvest service. Here are the steps:

1. Extract WMS and WFS GetCapabilities URLs from FGDC XML metadata and write them to a file.
2. Create a GeoNetwork Harvest Node from extracted GetCapabilities URLs and let GeoNetwork's OGC WMS/WFS harvester create ISO 19139 metadata from GetCapabilities response. Note: the resulting ISO 19139 metadata is only as good as the one in the GetCapabilities response. In addition, GetCapabilities responses do not include all fields necessary for minimum ISO 19139 metadata.
3. Copy some FGDC metadata entries (Title, Abstract, etc.) to newly created ISO 19139 metadata.

Our goal is to have working WMS and WFS metadata records in our CSW catalog that can be used to add the described services to analytical software such as ESRI's C-SW Client for ArcGIS Desktop. Following are the reasons why I am currently choosing this convoluted process instead of a direct FGDC to ISO 19139 metadata conversion:

Files: 

MultipartPostHandler.py_.txt

GeoNetwork offers a harvest service that, among other, follows a OGC WMS or WFS GetCapabilities URL and transforms the response into an ISO 19139 metadata record. Bear in mind that the resulting ISO metadata record is only as good as the GetCapabilities response and that the metadata can never be entirely ISO 19139 conformant without dummy values due to limitations of the OGC GetCapabilities schema.

GeoNetwork offers a handy user interface to add those "GeoNetwork Harvesting Nodes" and also exposes their functionality through a even handier XML service. See chapter 19.3 on "Harvesting Services" of the GeoNetwork opensource V 2.4 The Complete Manual. Following are my notes on creating an OGC harvesting node through GeoNetwork's harvesting service.

Tested with deegree-csw 2.3pre
Read the XSLT file for more information.

Attached is an example XSLT1 script to transform a WMS GetCapabilities 1.1.1 response to a CSW Insert transaction. The script is based on deegree's wms2iso19119.xsl (http://www.deegree.org/).
Note that currently it only supports WMS version 1.1.1 (<WMT_MS_Capabilities>) responses because it chokes on 1.3.0 (<WMS_Capabilities>) responses.

Files: 

etl_wms2csw_v3.py_.txt

Files: 

wms2iso19119_insert_XSLT1_07.xsl

Files: 

wms_getcapabilities_url.txt

... kind of sounds like overkill, but there's reason behind the confusion!

Imagine that you're trying to find some data. In the USGIN scheme, in order to find data you search through CSW services, which are catalogues full of ISO1939 metadata documents. Each of these documents points you to a dataset or service that is somewhere, hopefully online, for you to take a look at. These CSW services are like the card catalogues of the digital era.