Re: Sitemap(s) for tens of thousands of 'vanity' subdomains? (*.smugmug.com)

Look here for a PHP function to ping Google with a sitemap URI:
http://www.smart-it-consulting.com/article.htm?node=133&page=36
Check the official FAQ under "resubmissions" for the specs:
http://www.google.com/webmasters/sitemaps/docs/en/faq.html#a4
Sebastian