QScriptEngine engine;
QScriptValue object = engine.newObject();
object.setProperty("undeletableProperty", 42, QScriptValue::Undeletable);
object.setProperty("undeletableProperty", QScriptValue());
qWarning() << object.property("undeletableProperty").toString();

With Qt 4.5, the above program prints an empty string (because the property was deleted). Since 4.6, it prints "42" (the attempt to delete the property was ignored).
We should decide and clarify whether the property attributes only affect property access from the JS side (4.5), or from both JS and C++ (4.6).