Many have been reporting random rebooting problems with Google's Nexus S. It looks like Google has figured out what the cause of the problem is and, according to "Google Employee" Ry Guy, a fix should be coming within the next few weeks.
"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."
It's frustrating that Google's flagship phones (the Nexus One and the Nexus S) have been released with quite substantial bugs. The Nexus One, if you recall, had issues with switching between 3G and EDGE immediately after release. However, Google has done relatively well identifying and fixing problems in a timely fashion.