I had the same symptom – initial blank line ruining my feed – but with a different cause. At my host (Dreamhost) I had my domain set to work both with http://domain.com and http://www.domain.com, but with the former the blank line appeared and messed up the feed. It’s all mysterious Apache-fu to me, but once I had them set http://domain.com to redirect to http://www.domain.com everything was fine whichever way the feed URL was put in.
