// Change Author URL Slug
add_filter( 'query_vars', 'wpleet_rewrite_add_var' );
function wpleet_rewrite_add_var( $vars )
{
    $vars[] = 'user';
    return $vars;
}

add_rewrite_tag( '%user%', '([^&]+)' );
add_rewrite_rule(
    '^user/([^/]*)/?',
    'index.php?user=$matches[1]',
    'top'
);

add_action( 'template_redirect', 'wpleet_rewrite_catch' );
function wpleet_rewrite_catch()
{
    global $wp_query;

    if ( array_key_exists( 'user', $wp_query->query_vars ) ) {
        include (TEMPLATEPATH . '/user-profile.php');
        exit;
    }
}