AT&T locker is available on wp8 & can be downloaded from the WP Market
The app isn't very good on wp8 though. I'm having trouble syncing from the phone, especially in the background. One good thing is that for a limited time you can upgrade from the 5GB storage to 50GB storage for free. So, hopefully the app will get updated/improved. In the meantime, you can upload from a desktop.
Although this is an older thread, it is one of our most commonly viewed AT&T Locker threads. To ensure all of our customers are aware of the coming change to AT&T Locker, we wanted to place this updated information here.