On this page of Google help:
https://www.google.com/webmasters/tools/docs/en/sitemap-generator.html#submitting
Google mentions that there is a way to notify them of an updated sitemap using an HTTP request.
When you click the link, it takes you to this page:
http://www.google.com/support/webmasters/bin/answer.py?answer=34592&topic=8482&hl=en#ping
But there is no information on where to ping with what request.
Does anyone know what this address is and what parameters are required?