It's of little surprise that the Nexus S has been rooted and already has Clockworkmod recovery installed, which is what you see above. Just as we predicted, custom ROMs are right around the corner, folks. If you need help getting started, we've got bootloader and fastboot info in our Nexus S forums. [@Clockworkmod, Nexus S Forums]
Update: Rooting instructions can be found here.
Update #2: If you're having an issue with those instructions (many are) have a look here for the old fashioned way to get rooted.