function test(x) {
  new Ajax.Request('/ajax.php',{
    method:'get',
    parameters: {user: "register", namecheck: v},
    onSuccess: function(transport){
      var json = transport.responseText.evalJSON();
      return ((json.istaken == 0) ? false : true); // This is what I need to return from test(x)
    },
    onFailure: function(){
      alert('Something went wrong...');
      return false; // Or this, in case of an error
    }
  });
  return false;
}