There is a problem with the Drupal aggregator module stripping out HTML escape codes (i.e. < > & etc.) from RSS feeds when using PHP <= 5.2.6 with libxml2 >= 2.6.32.
This is due to an intentional change in the behaviour of libxml2 after version 2.6.32. Some sites suggest reverting to libxml2-2.6.30 - while this works as a temporary solution, it is no longer necessary or advisable.
PHP 5.2.7 or higher works with the new behaviour of libxml2 ( see: http://bugs.php.net/bug.php?id=45996 ). Simply upgrading PHP corrects the problem.