Have you been in the middle of an important call and your new Samsung Nexus S randomly rebooted? Google now has confirmed and been able to recreate the issue. Although this bug was reported a while ago, Google is now working with Samsung to address the issue. Google previously noted it being not an issue with the new Android OS 2.3 (Gingerbread) but a a manufacturer-related issue instead.
At this time, no fix to this bug has been resolved, but as you can see, Google is working at fixing the problem. Hopefully, we will see an update very soon.
We'll keep you informed as to the progress of this update.
Source: Android Central