Are there two different versions of the Nexus S, locked and unlocked?
No, the Nexus S is always unlocked, regardless of whether it is sold with a T-Mobile contract or not. This means that a consumer can remove the SIM card and replace it with any other SIM card from any other carrier. It is available with a 2-year T-Mobile contract or with no contract. When a consumer buys a subsidized ($199.99) phone, they will have to commit to a T-Mobile plan but even in that case the phone is unlocked.