file_get_contents() failed to open stream:

I'm trying to use file_get_contents() to grab a twitter feed, however I'm getting the following warning:

failed to open stream: HTTP request failed! HTTP/1.1 400 Bad Request

My code:

$feed = 'http://twitter.com/statuses/user_timeline.rss?screen_name=google&count=6';
$tweets = file_get_contents($feed);

I'm using Google just for the sake of testing. allow_url_fopen is enabled in my php.ini file.

Any idea what could be wrong?