Just went through the same problem where I was getting the XML Parsing Error: XML or text declaration not at start of entity error on my feeds (I have 3 WP blogs either ver 2.6.5 or 2.7b3) and I was able to solve the problem on all the blogs’ feeds by deleting space at the end of the wp-config.php file. Hopefully that will save someone else the headache of troubleshooting.
