badjedi - you likely have some whitespace after a closing '?>' tag (might want to check the RSSFeed.ss template to make sure its just not space in there).
Usually you'll just have to go through your code files and double check the ?> is the last thing - no whitespace, no spaces. I prefer removing the whole ?> at the end as its not required and it means issues like this don't crop up.
Both Will & Bruce's solutions work, but Will's is the 'correct' one...
1) because if core code is changed, when it is upgraded, the same changes must be made again. Changes to core code need to be carefully tracked to make sure they are not lost at the next upgrade.
2) because removing the XML declaration invalidates the file format, even if your browsers/reader can cope with it.
"XML documents SHOULD begin with an XML declaration which specifies the version of XML being used."
The simplest solution is to remove ?> from the end of ALL php files.
>> ...could not find the file with the offending white space.