Since Google XML Sitemaps v3.2.2 and WP v2.9 the sitemap plugin does not automatically my sitemaps when i publish a post. It just says:
Your sitemap is being refreshed at the moment. Depending on your blog size this might take some time! Due to limitations of the WordPress scheduler, it might take another 60 seconds until the build process is actually started.
Manual sitemap update works just fine.