Some percentage of Nexus S units in the field are afflicted with random reboots; there doesn't seem to be any rhyme or reason to when it happens, how often it happens, or what causes it -- but needless to say, a phone that could restart itself at any moment is not the best situation. After a few weeks of angry threads in the official Nexus S support forums, Google employee Ry Guy has chimed in with this encouraging tidbit:
So that's a good thing -- and in the meantime, we just hope you enjoy occasionally seeing the awesome Nexus boot animation by surprise."The initial results of our tests look very promising. At this rate we will most likely be rolling out an OTA to resolve this issue within one to two weeks. We really appreciate your patience as we do our final verifications on this fix before sending it out."