I prefer letting cron update sitemaps in the background, and at the end of the script I ping search engines to let them know it’s been updated:

   1  # Recreate sitemap goes here
   2  
   3  # Let search engines know about the update
   4  [ "http://www.google.com/webmasters/tools/ping?sitemap=http://xxx/sitemap.xml",
   5    "http://search.yahooapis.com/SiteExplorerService/V1/ping?sitemap=http://xxx/sitemap.xml",
   6    "http://submissions.ask.com/ping?sitemap=http://xxx/sitemap.xml",
   7    "http://webmaster.live.com/ping.aspx?siteMap=http://xxx/sitemap.xml" ].each do |url|
   8    open(url) do |f|
   9      if f.status[0] == "200"
  10        puts "Sitemap successfully submitted to #{url}"      
  11      else
  12        puts "Failed to submit sitemap to #{url}"
  13      end
  14    end
  15  end
  16  

More about sitemaps: http://en.wikipedia.org/wiki/Sitemaps