I've checked with other Dev Alpha users and effectively the app does show as available for download if you are using a T-Mobile SIM for data on the device.
We already know the players, over 50 carriers world wide. We already know the date: January 30 2013. As the date gets ever closer, we will begin seeing more and more information come out regarding which specific carriers are signing up to carry the BlackBerry 10 flag come January.
T-Mobile users will be happy to know there already is a "T-Mobile MyAccount" app in the works for BlackBerry 10. At this point, the app shows as "unavailable for this device" but it is a clear sign they are getting ready to carry RIM's next flagship devices. Would you switch carriers to grab a new BB10 device?