Very often illegal XML is returned by a call to http://api.twitter.com/1/statuses/user_timeline.rss . The value is <twitter:source> is not XML encoded.
E.g. <twitter:source> <a href="http://www.google.com/support/youtube/bin/answer.py? hl=en&answer=164577" rel="nofollow">Google</a> </twitter:source> It should be <twitter:source> <a href="http://www.google.com/support/youtube/bin/answer.py? hl=en&answer=164577" rel="nofollow">Google</a> </twitter:source>