The reason they removed the "Unlocked" from the T-Mobile version is probably so they can keep people who just want an unlocked phone from buying it since supplies are limited. Knowing there would be a shortage, T-Mobile may have asked Apple to do it so T-Mobile customers would have better access knowing full well it is unlocked.
I was thinking Apple removed the unlocked verbiage since THEY can't officially offer the Unlocked iPhone (do to inventory levels).
Since T-Mobile doesn't have contracts anymore, and all of their mobile phones have to be purchased unsubsidized, You would assume all of the phones they now sell are unlocked.
Also, i'm sure T-Mobile has to Buy/Sell X amount of iPhones a year to keep their contract with Apple.