I believe that what this is doing is sending all of the search engines to your a sitemap generating script rather than having them go through your site page-by-page.
It's good to question how that code was added, if you didn't add it would someone else have access to add it? If nobody else has access then you should check your access logs. You will also want to check every file that wasn't overwritten with the update to ensure no other code is potentially compromised.
I'm going to agree with Susan in regards to the code, you could alternatively use the robots.txt file instead.