I was actually a bit surprised by it as well, but in reading a few articles on Google News, a "common myths" about the prize came up. One of the myths was something like "You have to have achieved a major accomplishment towards international relations [or something like that] before winning the Peace Prize." The correction was that the prize was often given to provide encouragement to people who have bold visions for peace to follow through with their plans.
That being said, there are people who could have probably benefited from the prize more than Obama, at least in terms of their peace work. Getting the prize as someone who is being jailed can bring attention to your plight. Giving it to people working to change draconian regimes can help encourage the regimes to improve more for the PR. And so on.
As much as there are huge challenges for Obama, especially considering the vast difference between the new administration's approach to international relations and that of the neo-cons (and Clinton, really), he didn't really need it. I do hope he lives up to the honour, and I say this as a person who voted for him and who still supports him.
ETA: The Peace prize is almost a way that the committee can bring the world's attention to something that desperately needs attention. Obama doesn't need it. It would be one thing if he had done something like found lasting solutions in the Middle East or actually figured out a way to contain the nuclear threat, but he hasn't yet. Save the encouragement for those who are risking their lives. Reward leaders after they've achieved something that justifies the honour.
Just my thoughts on it.
