I dont know for sure -- but my guess is all the Nexus S phones are unlocked.
According to this thread, the Nexus One was unlocked with a contract:
I wouldn't take the Best Buy FAQ as gospel. For example one of the question is:
Are there two different versions of the Nexus S, locked and unlocked?
No, there is only one Nexus S. It is available locked with a 2-year T-Mobile contract or unlocked with no contract.
The answer is no, but then they go and say there is locked and an unlocked version. Did a 10 year old write this or what?