Update x2: The update is rolling out again as of April 8th. Hopefully it'll last this time.
Update: Turns out that T-Mo recently halted the update. At this point there's no word on why it was delayed, but the carrier updated the support page to reflect to change, noting that it is "paused until further notice." We'll let you know when it starts rolling out again.
If you picked up the T-Mobile-branded version of Samsung's newest 10-inch slate that doesn't have "note" in the name, you may want to head into the Settings menu and tap the update button, as T-Mo has quietly started pushing the Android 4.1.2 update to the device.