Illegal characters such as ampersands are not escaped in the CiviEvent RSS feed: http://demo.civicrm.org/drupal/civicrm/event/ical?reset=1&page=1&rss=1
http://feedvalidator.org describes this as not well-formed XML. It's acceptable for the RSS feed to not be valid XML, but it should really be well-formed, as the spec encourages XML processors to refuse to handle non-well-formed XML. It may be necessary for all HTML tags to be stripped if the description is not well-formed XML, unless CiviCRM already includes a library which can balance badly-placed tags.
Difference between well-formed and valid XML: http://www.xml.com/pub/a/98/10/guide0.html?page=4