I hope that this is not someones mistake, but as we can see at Apple Store Mexico, the actual iPhone 3G S is unlocked. Mexico is still on the list for next iPhone 4 shipping and for now, nobody knows when they will be available for purchase.
I'm shure that it will stay unlocked. Previous version 3G S 8GB has attractive price of 8,199 Mexican pesos (free shipping, aprox. 650 US$), which for unlocked version is not so bad.