PHP
	if (isset($_GET['verificarlogin']))
	{

		$Login = $_POST['login'];

		$sql = $Site->sql('SELECT * FROM '.$Db_Site.'.usuarios WHERE login="'.$Login.'"') or die (mysql_error());

		$var = array();

		if(mysql_num_rows($sql) > 0)
		{
			$var['existe'] = 1;
		}
		elseif(mysql_num_rows($sql) < 1)
		{
			$var['existe'] = 0;
		}

		echo json_encode($var);

	}

JAVA
		jQuery('input[name=login]').blur(function (){
			jQuery.ajax({
				url: '?verificarlogin',
				dataType: 'json',
				success: function (data) {
					alert (data.existe);
				}
			});
		});