He was awarded the price for his use of diplomacy instead of provoking more new wars. There was a lot of controversy regarding the decision, but to say that it lost all credibility is simply not true.
He had had zero time to do any of that, being elected just a year before.
Cough. According to Wikipedia[1] - "Nominations for the 2009 Nobel Peace Prize closed just 11 days after Obama took office." That must have been a busy 11 days doing diplomacy...