Although it started to take pre-orders after the other three major carriers in the US, it appears that T-Mobile is the first one to ship the latest LG flagship. Official sales are scheduled to begin at the end of next week (Verizon Wireless) and on April 6 (the others).
http://www.notebookcheck.net/T-Mobile-already-shipping-the-LG-G6.207964.0.html