How can I enable automatic linking to Twitter usernames in my WordPress posts or comments?

function wpsc_content_twitter_mention($content) {
return preg_replace('/([^a-zA-Z0-9-_&])@([0-9a-zA-Z_]+)/', "$1@$2", $content);
}
add_filter('the_content', 'wpsc_content_twitter_mention');
add_filter('comment_text', 'wpsc_content_twitter_mention');