You're right, Live Search does not, right now, offer the ability to publish sitemaps via ping or webmaster tools; additionally we do not necessarily take into account sitemaps listed in robots.txt files right at this moment. However, this will change very soon. As some of you may have already discovered, we've started considering sitemaps in robots.txt and will extend this to most sites within the next few months. The other methods you've mentioned are certainly on our radar for the near future and we'll be sure to communicate related developments as they happen.
After collaborating with Google and Yahoo to define the SiteMaps protocol, we're committed to taking and using the content of sitemaps. Our latest announcements will continue to be available at: [blogs.msdn.com...] .
A final note: If you use autodiscovery for sitemaps through your robots.txt today, you'll automatically benefit when Live Search and other search engines enable support for this feature -- so we strongly encourage it!