"Ask has since documented sitemap support here:
While the ping mechanism does work, I haven't seen them actually crawl the file, but that could be attributed to their low overall crawling rate.
Before embracing sitemaps auto-discovery, one should ask the question: is it sensible to facilitate content scraping by providing a list of all of your URLs to the world? I believe it is wiser to stick with manual notification via Google's Webmaster Dashboard and Yahoo!'s Site Explorer. To the extent there are few major search engines, manual pinging is not a major issue for the others, which for now is limited to Ask as Microsoft has not yet begun supporting sitemaps."
How to add sitemap autodiscovery?
Once you have prepared the sitemap, add the sitemap auto-discovery directive to robots.txt as follows:
SITEMAP: http://www.the URL of your sitemap here.xml
The sitemap location should be the full sitemap URL. Alternatively, you can also submit your sitemap through the ping URL:
http://submissions.ask.com/ping?sitemap=http%3A//www.the URL of your sitemap here.xml
Please note that sitemap submissions do not guarantee the indexing of URLs. To learn more about the protocol, please visit the Sitemaps web site at http://www.sitemaps.org.
For more updates and news on Ask.com's sitemaps autodiscovery, watch this space.