You waited and waited and finally got a Nexus S. Given the history of other phones you figured it would be a week or so before you could unlock the Nexus S, and another week before you could root it.
Wrong on both counts! The phone was released today, and already we can OEM unlock and root it!
To get started you're going to need a Nexus S, a microUSB cable, and a computer with the latest Android SDK installed on it.
This is also called "unlocking the bootloader". To do so, reboot your phone into bootloader/fastboot mode by turning off the phone, then holding Volume Up and the Power button.
These steps are provided as-is without warranty (express or implied), and may turn your brand-new phone into a brick. Continue at your own risk.
Once you're booted up, connect the phone to your computer via the microUSB cable.
Open a command prompt (with administrative privileges)
Navigate to the tools folder inside the folder in which you installed the Android SDK (similar to "C:program filesAndroid SDKtools")
fastboot devices(enter) and make sure your device is listed
fastboot oem unlock(enter)
Download the recovery-clockwork-herring.img recovery image and copy it into your tools folder
fastboot flash recovery recovery-clockwork-herring.img(enter)
adb reboot recovery(enter) to reboot into the new recovery image
Install the Superuser .zip from this thread
Koush says that support for the Nexus S is coming soon to his ROM Manager app which will make future tasks grotesquely simple.
UPDATE: Some people are reporting that not all superuser permissions are available, or are not behaving as expected. You may want to stick with only OEM unlocking for the time being.
Source: Koush on XDA-Developers