Several reports came from Google Groups starting on May 27th. All have reported that no matter what they do, the delete function does not actually remove the sitemap from the associate site.
The first report said:
I've tried to delete the RSS feed "sitemap" from webmaster tools, but it doesn't seem to go - te page reloads - but both sitemaps are still showing?
Googler, Susan Moskwa, replied saying they are looking into the issue:
Thanks for reporting this issue. Our team is looking into it.
FYI no need to worry, since those extra Sitemaps shouldn't cause you any problems.
On a similar note, a WebmasterWorld thread reports that Google seemed to have merged two of his sitemaps files into the same domain. I.e. He set up profiles for subfolders, so he can manage each subfolder, which Google recommends. Google seemed to have automatically merged those two sitemaps under a single site. Which begs the question, how do you delete part of your site using Webmaster Tools?
Finally, a DigitalPoint Forums thread is reporting a backlink update in the link tool within Google Webmaster Tools. As an FYI, that tool does update every few weeks.