Search by meta_query

You should look into meta_query:

$args = array(
'meta_query' => array(
        'key' => 'event_start_timestamp', // the name of the meta
        'value' => '11/21/2011', // the value to search for
        'compare' => '>=', //search for start dates bigger or equal to value
    )
); 
$query = new WP_Query( $args );