I have a 10-digit number (no letters) and it works. I get the same information everyone else is getting for the 6s (ships on 9/24). I ordered via the Costco deal on Tuesday and got my order number by calling T-Mobile yesterday.
nice! do you mind sharing the first 4 digits so I can update the first post?
out of curiosity: what exactly was the costco deal? I heard about the $100 rebate, does that combine with T-mobile's JOD $125 discount?