An unlocked GSM version of the iPhone 5s has been released by Apple on its online store for the United States including the same exact capabilities of the non-unlocked models, but does not include a SIM card.
The unlocked model will let you insert a nano-SIM of your choosing. You still have the option of shipping your unlocked model with a T-Mobile nano-SIM preinstalled which you can then activate for use on T-Mobile’s network.
A SIM-free version of the iPhone 5c has been available since its launch in September. Many international countries have had availability of the fully unlocked iPhone 5s for several weeks.