Looks like T-Mobile is preparing their app for the official launch of BlackBerry 10 devices in the USA as they have just released an update for the T-Mobile MyAccount app in BlackBerry World.
As a T-Mobile user myself with a Z10, I wouldn't get your hopes up just yet, as the app works worse than before the update. Now I can't even login to see my account info anymore. Let's hope they weed out these bugs before the official launch in stores. Also, this is an Android port.
App description: T-Mobile My Account is the easiest and fastest way to connect with T-Mobile on your phone. From one easy to use application, you can view and manage your T-Mobile account, quickly access Help and How-to’s for your phone, receive customized notifications, and learn more about T-Mobile services. You can also set alerts and reminders to help you optimize your phone’s battery and performance.This is a FREE service; there are no usage charges (including data) to use the application or view account usage information.