Google is preparing to phase out the OAI-PMH (Open Archives Initiative Protocol for Metadata Harvesting) from Google SiteMaps which was used as an inter-operable framework based on metadata harvesting.
According to Webmaster Central Blog, Google is finding it difficult to keep this service operational, as the number of resources required to operate it far outweighs the information gathered from it. As of now less than 200 websites across the globe are using the 'Open Archives Initiative Protocol for Metadata Harvesting' for Google SiteMaps, which in itself is a very negligible figure.
Google is encouraging all Webmasters to reach out to the OAI-PMH alternative that will be, the standard XML Sitemap format support that Google would be providing to the Webmasters by May 2008. All those site owners who are availing the OAI-PMH services for Google SiteMaps are in the process of being informed by Google. The XML Sitemap format is compatible with all search engines and Google has assured site owners that the search crawlers would be able to locate their content as soon the Webmasters make it available and visible.