strtolower

Wrapping everything in strtolower should work:

<title><?php
global $page, $paged;

echo strtolower(wp_title( '|', false, 'right' ));

echo strtolower(get_bloginfo( 'name' ));

$site_description = get_bloginfo( 'description', 'display' );
if ( $site_description && ( is_home() || is_front_page() ) )
echo strtolower(" | $site_description");

if ( $paged >= 2 || $page >= 2 )
echo strtolower(' | ' . sprintf( __( 'Page %s', 'twentyeleven' ), max( $paged, $page ) ));

?></title>