in the changes.txt it says:
but setAmbientLight() is not declared in the scene manager interface.- The scene manager now sets an ambient light color when calling ISceneManager::drawAll().
You can influence this by calling ISceneManager::setAmbientLight(). That light color
is also stored when saving .irr files via ISceneManager::saveScene().
(however it is CSceneManager.cpp & .h )
just for your info.