Google Nexus S already rooted0
Anyway, if you are in a rooting mood, here are the instructions as posted by XDA-Developers:
"First, reboot into your bootloader/fastboot mode. (Volume + Power Up, or adb reboot bootloader)
fastboot oem unlock
Then, install this recovery:
fastboot flash recovery recovery-clockwork-herring.img
Then boot into Clockwork recovery and install the Superuser zip file (adb reboot recovery, or from fastboot choose recovery):
To install a rooted/insecure boot image (adb has root):
fastboot flash boot rootboot.img
ROM Manager support is coming shortly."
Good luck, lads (and ladies?)!
source: XDA-Developers via Redmond Pie