Troubleshooting why my Android phone is rebooting (Nexus S)
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:

aLogCat