No surprises here, but T-Mobile USA’s Xperia Z (C6606) will have a bootloader that cannot be unlocked. This means users who buy the handset will be limited when playing with custom ROMs, although there are some ROMs that work on handsets with locked bootloaders.
Even if T-Mobile allowed you to unlock the bootloader reasons against it include voiding your warranty and wiping the DRM keys from TrackID and the Mobile Bravia Engine. As mentioned earlier, the news is no surprise as even in the UK most Xperia devices bought directly from a carrier will have a bootloader that cannot be unlocked.
It was always going to be a similar case with T-Mobile USA. The confirmation below is from a T-Mobile branded Xperia Z (C6606) that XDA member HoOnZ tested at a Sony Style store.