I'm guessing this happened on either late night 4/8/13 or early morning 4/9/13:
BEST BUY no longer has the ONE for T-MOBILE. They've totally removed it from the website,only showing the SPRINT & AT&T models, as if the T-MOBILE variant was never there. Not even available as an unactivated (unsubsidized) purchase
One of two possible explanations :
Sold out & no longer taking orders for the T-MOBILE version in the future..
Or, took it down & refunded those who pre-ordered, as the original orders were w/the classic plans, which are no longer offered by T-MOBILE.
Anyone happen to order the ONE on T-MOBILE via BEST BUY & have some information on the subject?
Found a partial answer,regarding T-MOBILE classic plans:
Best Buy Internal Doc Says Simple Choice Plans Coming At End Of "Fiscal Year", Classic Plans Still Available | TmoNewsTmoNews
Now,to find out about future stock for the ONE one T-MOBILE @ BEST BUY.......
(an SKU search  on BEST BUY's site turned up nothing)
EDIT PART DEUX:
Found a sign-up for it on BEST BUY's e-BAY store: