I was surprised when I read about it this morning, but it makes sense to me. The Nobel prize has become somewhat of a lifetime achievement award, but it was originally established as an award for those individuals who've made the greatest contributions to humanity in the past year. In the past year, Obama has reversed international opinion regarding the only current global superpower, he's reversed the Bush-era doctrine of preemptive war and reestablished the importance of diplomacy and cooperation in US foreign policy. He's also started a major fight for health care reform, which is critically needed by tens of millions of people in the US, and he's the first black president of the US, which has historical significance and has a present-day impact on international opinion of the US.
Obama hasn't sweated blood and tears in the way that many previous peace prize winners have, but the Nobel prize is for results, not the personal effort from an individual, and as the president of the US, Obama can get results with comparative ease.