The official launch date for Samsung's new Galaxy S7 (and Edge variant) is supposed to be March 11th, but T-Mobile cares not for launch dates. The Uncarrier has started shipping devices to those who pre-ordered, and they should arrive in the next few days.
This is actually the second year in a row that T-Mobile has shipped the new Galaxy device early. Last year, Tmo beat all the other carriers by more than a week. It is unclear if Samsung specifically allows T-Mobile to do this or if they're just having an "eff this, we're doing it live" moment.
T-Mobile reps are telling callers that the "official" start of shipping is today, but a few shipping notifications have already gone out. Some of the shipping dates listed are as early as last Friday, so devices might even show up today or tomorrow for those who paid for faster shipping.