It seems to happen when I'm using Pandora. But then again, I'm always using Pandora. It also reboots so quickly that sometimes, I do not notice.
Is there a way to see what caused the last reboot?
Engadget has just reported that the Nexus S is having issues with random reboots (http://www.engadget.com/2011/02/15/nexus-s-suffering-from-random-data-loss-reboots/) and apparently fixes are coming.
To see the log you can get the following app: