According to Nobel's will, the Peace Prize should be awarded "to the person who shall have done the most or the best work for fraternity between nations, for the abolition or reduction of standing armies and for the holding and promotion of peace congresses."
Alfred Nobel's will stated that the prize should be awarded by a committee of five people elected by the Norwegian Parliament. Norway and Sweden were at that time still in union, and with Sweden responsible for all foreign policy, Nobel felt that the prize might be less subject to political corruption if awarded by Norway.
Barack Obama is trying to make the world nuclear-free (that is Nobel worthy), but how on Earth can you give it to the President of a country that hostilely invaded two nations and is currently occupying those nations? Every other day it seems we hear that the President ordered another strike into.
I'm not saying that he is unjustified for his military actions, only that it should render him disqualified for receiving the award.
That other warmongers (Henry Kissinger) won the award is irrelevant. Those were mistakes too.
There is an argument that eliminating Bush's missile defense shield in Europe for a more "comprehensive" deterrent strategy aimed at the Iranian threat significantly reduced the likelihood of the US and Russia going to war. That likelihood was very slim anyway, and despite that the President hasn't done anything to "render the greatest service to the cause of international fraternity, in the suppression or reduction of standing armies, or in the establishment or furtherance of peace congresses." How could he have he has been President for less than a year.
In fact, it is probable that he will increase the number of troops in Afghanistan soon. I strongly believe he did not deserve the award he won today, but still I pray he uses it as a tool to help aid his quest for a nuclear-free world.
EDIT: Before I respond to the many comments below, I would like to give another reason he is undeserving; his failure to publicly condemn the former President and others responsible for torture. That he has put an end to torture is a good thing, but not laudable. The US should not torture in the first place.
EDIT II: Also, the fact that Kissinger won the prize isn't really a good counter. Wilson and Kissinger did not deserve the award either. Precedents matter, tell Mr. Obama this regarding torture prosecutions.
EDIT III: For all the ad hominems against me, Mr. Obama agrees with me! He certainly is humble. He also shares my opinion that he is undeserving of the award. He also shares my desire that the award will enable him to push forward the causes he is pushing for (a nuclear-free world would be fantastic). Here's Hoping.
EDIT IV: Michael Moore, more articulately, agrees with me. Congratulations, NOW EARN IT!!