Google announced that they have dropped this feature. Sitemaps are still going to work, but will of course be slower.
Websites can "Ping" search engines to notify them when new content is available. For my WordPress instance I use a sitemap generator plugin that also uses this method to notify Google, Yahoo, Bing etc. When I publish a new post it is available there instantly.
I would assume that the StackExchange network uses a similar function.
Quote from the plugin faq:
This plugin can automatically notify Google and YAHOO when the content
of your blog changes. This service is free to use, YAHOO just requires
an API key which can be freely obtained here. After the search engines
recieved the "ping" they may come and crawl your site again. Since the
sitemap files contain the last change of every post or page, the
spiders should just retrieve the changed ones and save your traffic.