One of my questions is, What phones would the application store be available for? T-Mobile USA has BlackBerry devices, Sidekick devices, Windows Mobile devices, Java-based devices, and custom environments from Motorola, Nokia, and Samsung. They were also one of the first to support the Open Handset Alliance (aka Android) and may be the first carrier to launch with an HTC Android device (I think they really need to be in order to bring subscribers to their network). The experience could vary widely depending on the mobile operating system that the applications would be available on and I think they may focus on the higher end smartphone market where people tend to buy 3rd party applications rather than just ringtones. Apple makes it look easy, but they have just the iPhone to support and it has a large display and powerful operating system to power the application store on the device itself. The T-Mobile store may or may not be available on the device in addition to a desktop access point.
There are other software stores, like Handango, that power other carrier's mobile application stores on devices like Windows Mobile, but I don't think there is a good cross platform store yet available and this could be where T-Mobile steps up and differentiates itself. This will not be an easy task and adding something like this will significantly increase the customer service demands as people buy applications and have issues.
There is a T-Mobile devPartner community site that looks well designed and has lots of information and tips for developers looking to support T-Mobile's devices. The site mentions the t-zones catalog as a place to showcase applications and also states, "In the coming weeks, T-Mobile will be offering new ways to go to market." I look forward to seeing what T-Mobile has to offer in the way of application distribution.