The only hacker who signed up to beat Android 2.3 in Nexus S is Jon Oberheide. The reason he resigned may be quite painful because he have chance to get U.S. $15,000 prize.
He did not expect that to qualify to participate in contests with prizes of thousands of U.S. dollars. So, when he found a weakness in the Android before the contest took place, he immediately notify Google to get a U.S. $1,337 dollar reward promised by the program that was designed by Google. Well, prior to the contest progresses, the weakness had been corrected by Google.