I think the propagation of the human race through space is a natural consequence of technological progression under the assumption that it does not annihilate itself before it is capable of doing so. As Vollstrecker already pointed out living space is a limited resource. Though I believe that the human population will stabilize somewhat logarithmically, the idea of terraforming and living on other planets is certainly appealing.
Whether or not terraforming a planet is ethical is a difficult question to answer. Most people seem to agree that if terraforming eventuated the annihilation of an exoecosystem then it would be unethical. As a whole, however, I think that the intrinsic xenophobia and selfishness of the human species would inevitably lead to such an annihilation given that our technology was significantly superior. If what society considers acceptable is also considered ethical then terraforming another planet is certainly ethical; a direct consequence of human selfishness.
I personally find the idea of terraforming another planet with life on it appalling. Human beings are like a plague to our own planet's ecosystem. I certainly hope that the plague never becomes infectious.
Even otherwise, I'm not sure I'm entirely comfortable with the idea of terraforming a planet. I find that there is inherent beauty in the way nature set things up. The human plague would only tarnish this beauty, with or without life. However, the spread of the human plague is a necessity for the progression of science. In my personal opinion, our quest for knowledge transcends any question of ethics unless we are harming living beings.
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