This functions gets the first post with the specified key and value in the postmeta table:

function getPostIdByMetaKeyAndValue($key, $value) {
	global $wpdb;
	$meta = $wpdb->get_results( $wpdb->prepare( "SELECT * FROM ".$wpdb->postmeta." WHERE meta_key=%s AND meta_value=%s", $key, $value ) );
	if (is_array($meta) && !empty($meta) && isset($meta[0])) {
		$meta = $meta[0];
	}		
	if (is_object($meta)) {
		return $meta->post_id;
	}
	else {
		return false;
	}
}

Links

• $wpdb – WordPress database access abstraction class.
• Ultimate Guide to meta_query