Andi disagreed with my use of "the Void" in this post, so I poked around a bit to see just what the relation is of the Void to Nirvana. Like I said, I think there may be some sectarian differences on this, but there's an interesting collections of links on this page, on a Tibetan Buddhist site. Here is a relevant bit about voidness, enlightenment and compassion:
There are two ways integrate absolute and relative spirits of enlightenment. When you develop the absolute spirit, you experience the voidness of all existence; at that very moment of experience of voidness, you will develop intense compassion for all beings, since they are not negated by the experience of voidness. When you develop this relative spirit of enlightenment, this profound compassion for all beings, you will simultaneously feel the nonseperation of self and others. Appearances are like a magicianís illusions; they are actually devoid of intrinsic reality.
When you have successfully developed this integrated realization of the two kinds of spirit of enlightenment, you have correctly entered the path of the Universal Vehicle. By cultivating that realization, you will perfect your meditation and will naturally obtain the Form and Truth Bodies of Buddhahood. The Truth Body arises from voidness, the absolute spirit of enlightenment. The Form Body arises from compassion, the relative spirit of enlightenment. From the cultivation of the indivisible two kinds of spirit of enlightenment, you will reach the indivisible Truth and Form Bodies of Buddhahood.