Hi,
WP's xml export doesn't currently escape the conents of many fields, including the comment author. If those fields include named HTML entities, that means it's invalid XML. The importer handles it just fine, but some browsers will complain with an error or refuse to download the export file if the XML doesn't validate.
Attached is an example of the problem output, and a patch that uses CDATA escaping on the comment author field. Other fields could be escaped too, but I've limited the change to the one that I've seen cause a problem in the wild.
On the import side, get_tag() will accept CDATA on any field now. It should retain backwards compatibility with export files created prior to this patch.