deadsea - 9:50 am on Jun 30, 2011 (gmt 0)
I do mega sites. Even on mega sites, the value of the xml sitemap is limited if your site has a crawl-able hierarchy.
Currently my XML sitemaps have every crawlable url in them. On some sites we use the priority field based on how good the page should be at ranking for keywords.
I'm thinking about trying something new. I'm thinking about only putting new pages and pages that have been updated since they were last crawled in the sitemap. With any luck this would let Googlebot focus its resources on the places that it actually needs to be crawling. I'd see boosts for QDF searches. Maybe Googlebot would even stop crawling some of my pages that rarely change multiple times a day.