This question is rather important to some people and less to others. The opinion varies from denomination to denomination. Conservative non-denominational evangelicals usually tend to think that our planet is the only planet with life in the universe. Answers In Genesis holds this view, of course they go so far as to claim life on other planets is based on "evolutionary" and "humanistic" assumptions (yes they can tell the difference between evolution and Secular Humanism, they simply choose not to I guess). On the other side of the extreme the Roman Catholic Church has embraced the idea of extraterrestrial life, they even go as far to say that space would be a missionfield to spread the gospel to other civilizations.
My personal belief is that there is life on other planets, possibly even intelligent life (which sometimes I wonder if it is yet to arise on this planet). There is nothing unscientific about the idea of life on other planets, the main question when it comes to Christianity is a theological one. These intelligent, humanlike aliens if they exist would have freewill. Thus they would have the ability to sin, and thus they would need Christ to save them. The problem is that Christ only died for humans, unless God has a radically different plan for them or he decided to just let them all burn in hell (which I think is very unlikely) Christ would have to visit each planet and die for each races sins. if we are talking unintelligent (or the other term presapient) life then the problem evaporates. The Pheonix probe which landed on Mars on March 25 is searching for water, and life on Mars (microbial life of course). They have even discovered something which might be ice on the surface. This is no problem for Christians. God simply also created life on Mars. Intelligent life isn't necessarily a problem either.
Now if Christ died for all sapient beings in the universe on earth. This problem would not be as bad. It could be that God created all sapient life in the universe about the same time (note this does necessarily mean special creation). At first they were all perfect, then Satan fell from grace and went about causing thousands if not millions or trillions of races to fall from perfection (although some of them may fallen simply because they made the wrong choice and Satan had nothing to do with it). Some would fall but others would resist temptation and right now would be perfect Utopian societies where there is no death, no suffering, no injustice, and no misery. Then there would be their fallen counterparts which would be much like the human race. Being capable of Godlike feats (such as science, humanitarianism, and contact with the creator of the universe) and at the same time being capable of unspeakable evil (such as genocide, rape, etc.). And when Christ died they now all have chance. It might even be humanities responsibility to bring the gospel to the universe, although I would count on it. No doubt God if he indeed exists has revealed himself to all these races and told them of Christs suffering on the cross. Now if Jesus died for each of them individually there are two options; if he did it simultaneously then it would be virtually no different except every culture would have a depiction of Jesus the savior. If he didn't do it all at once then it seems like it would be almost a cycle that all races go through independently. First they would be born unfallen and without flaws, then if they fall into sin they would be a fallen race. Then Christ would come, die on their planet (or at least one of their planets) then they would be a "semi-redeemed" race which is in the process of being restored. Finally there is a Redeemed race; which is back to prefall perfection being ruled directly by God. This idea seems far to De-Chardinish for my taste. Most likely the other view that Christ died simultaneously for the sins all race had or ever would commit is correct, the reason being that there is no scriptural support for a cyclic pattern in the redemption of sapient beings. There are also Christians who think aliens are really demons, well I don't think so, they would probably be on the same level as us.
At the moment this is not a particularly important question since no life in outer space has been discovered. But it is still interesting to talk about.