I ran into a problem that seems to come up occasionally on the WordPress support forum – blank lines at the beginning of the RSS feed XML file for the site which causes problems since it’s no longer valid XML. A quick look at the support forum pointed me in the right direction. It turned out to be blank lines following the final ?> line in the functions.php I had added to my custom theme to add widget support to the sidebar. This file gets loaded and parsed for every request to the blog, including for the RSS feed, and the blank line is actually seen as an intended part of the output – like the regular HTML you see in PHP files. Removing the blank line sorted the problem out.