I'm not sure if people have seen it yet, but Google have unveiled a system for submitting site maps to them, as crawl hints; the idea is that the site map can hint at page importance, edit frequency, and last edit date, so they can crawl more sensibly.
Google FAQ - https://www.google.com/webmasters/sitemaps/docs/en/faq.html My Sitemap - http://nottingham.openguides.org/sitemap.xml My (not very pretty) script to create said sitemap - http://www.jimbo.org.uk/code/make_sitemap.txt
It's not documented or commented yet, and I'm not sure what best to do with the <priority> and <changefreq> elements yet, but it mostly seems to have worked thus far.
Hope this is useful to someone!
Cheers, James