In case you havenīt yet: think about using two ambient terms instead of one for your models and blend between them based on the normals y value. This way you can fake some kind of sky light and indirect illumination from the ground and it is at no real performance cost.
One could also look up the ambient color from a blurred version of the skycube itself, based on normals, that way it always fits the visible sky and more or less the ground, given the skycube has one.
