One thing that has to be decided is what the word "liberal" means in a political context (since that is the context in which the question was asked). It would be nice to look at American politics, but the more you look into that, the more the lines blur. What is an American liberal or conservative? Is it a raging socialist vs. a mouth-foaming Tea Bagger? Can we reduce the political parties to less government vs. more government?
For the purposes of this post, I am going to take liberal to mean a belief in reform, progress, equality in a broad sense, environmentalism, and moderate to significant government intervention to achieve social cohesion; conservative will, for me, represent a disinterest in change (i.e. maintaining traditional values), individual liberty over sanctioned equality, and valuing profit and capitalism over people and the environment. These are all debatable, but this is the closest I can get to addressing the liberal vs. conservative argument in SF without bringing in irrelevant stuff. For example, while liberals are typically for abortion (if not in every form, then at least on a basic level) and conservatives are typically against it, it isn't an issue that regularly appears within SF (I can't even think of an example right now). Other liberal/conservative issues are the same, and so I'm not including them in the definition.
Now to the answer:
Having read and watched loads of science fiction books and movies, and dabbled in writing the stuff myself, I consider myself well-versed in SF. Yet, when I think about this question it occurs to me that the liberal/conservative issue has never seemed to be, well, an issue. I'll read most anything in SF, and have wandered around enough in the SF landscape to be considered an SF slut.
But looking back at what has been applauded by the SF community, or enjoyed fervently by me, it does seem that the majority of SF stories are to the left of the political scale. James Cameron's Avatar, whether great or terrible, is undoubtedly liberal; its messages range from environmental to racial and so on, with the bad guys clearly marked as the wicked militaristic capitalists, and the good guys the soon-to-be-tree-hugging whiteys (and the giant smurfs, obviously).
Examples of similar liberal leanings exist throughout SF film: Star Wars, WALL-E, and so on. There are exceptions: Metropolis both critiques industrialization and scientific/social progress (after all, the workers' revolt in the end leads to the workers' city being destroyed, which is not exactly a positive for the anti-industrialization crowd); Aliens isn't altogether clear what it is (on the one hand it's about the evils of the company/corporation, but on the other it's about the gung-ho "shoot before asking" mentality that exemplifies the rather conservative old west more so than the probable more liberal future); District 9 only sort of supports liberal anti-corporate interests in the end, but the rest is only liberal if you don't agree with the point of view being presented (which is exactly what happens in the real world in Africa, in terms of corporations dictating what goes on); and so on.
Literature is no different. Regularly SF novels play the liberal vs. conservative card (defined generally by the present-day political climate). Edward Willett's novels Marsegura and Terra Insegura pit genetically augmented fish people against a rabidly religious post-apocalypse Earth; issues of race, religion, tradition, and so on appear in the novel and, despite some ambiguities towards the end of the second book, present liberal values as the "good" ones and conservative values (albeit of the most extreme kind) as the "bad" ones. There are certainly plenty of other examples, most less obvious than might be found in Willett's work or in the work of the infamous Kim Stanley Robinson.
Some examples of conservative SF do exist, though: Frankenstein (against unfettered scientific progress; 1984 (an easy choice, since it is a critique of extremist liberalism); some of Heinlein's work (entrepreneurs fighting government restriction, and so on); and many more.
The interesting thing about SF literature is that its political leanings are somewhat easily isolated by genre. Military SF, for example, tends to be rather conservative compared to other forms of SF, mostly notably because the military often is perceived as conservative (even if that perception is inaccurate). Most of this is hearsay, to be honest, since I am not altogether familiar with military SF as a reader (just as an academic).
But (and this is a big but) none of this proves that SF is inherently liberal. It does demonstrate that much--perhaps most--SF is liberal, sure, but that is an entirely different thing that what is implied in the question: that a liberal view of the world is an essential characteristic of SF. There are only a handful of things that I would be comfortable saying are inherent to SF, but a liberal view is not one of them. The other side of this is somewhat more complicated: even when liberal views are present and emphasized, they are often in league with conservative values. This seems to reflect the wishy-washy way in which Americans deal with politics, because most of us are a collection of liberal and conservative ideals, with one seemingly more pronounced than the other. SF, in my opinion, is less about the flaws of any particular political position or belief than about human flaws, and if you think about the extreme future of any human flaw, you'll end up with something worth critiquing.
I think the much more interesting part of all of this is the relative paucity of conservative SF in film and literature. Are conservatives less able or less inclined to think about their own future? Are writers more often than not of the liberal persuasion? You'd think that the answer to these two questions would be a resounding "no," but here I am...wondering.
So, the question I'm going to throw out to all of you is the same question that was asked of me: is science fiction inherently liberal?
Let me know what you think in the comments.
If you'd like to ask me a question about science fiction, fantasy, books, writing, or whatever (anonymously, even), feel free to ask on my Formspring page or via email at arconna[at]yahoo[dot]com (or as a comment).