Nexus S has been achieved on the phone's day of release, and full instructions for replicating it are available at the link below. The Nexus S is the proud and so far only carrier of Google's latest and greatest Android software, so you're basically getting unrestricted access to the very best Mountain View can offer. Hit that source link. Do it.
Update: An Android team member has written in to remind us that the Nexus S, like the Nexus One before it, is designed with a user-unlockable bootloader out of the box -- it's not a magic trick. Good to know they're keeping the tradition going.