I'm also interested. I'm not on the Upgrade Program yet, but would like to sign up as a T-Mobile subscriber.
On Tuesday (before launch day), I called Apple and they told me to go to the store. I did, and they said I could only sign up when they have stock. I asked about getting a 6 on the Upgrade Program to tide me over, but that's no longer an option. With no way to reserve an iPhone 7 for the Upgrade Program and T-Mobile, the store might have stock when I call, but none when I arrive. What a pain.
Today, I did a chat with Apple sales. They said the program is fine and will work when they have stock. I'm not sure I believe them.
Has anybody signed up as a new Upgrade Program member with a T-Mobile iPhone 7? I'm skeptical.