The app is designed by Windows Phone developer Rudy Huyn, known for his knock offs of popular iOS and Android apps for the Windows Phone platform, and Belfiore's tweet says that the beta version of the app is about a week away. The tweet also revealed that the app will be available for English devices that carry 1GB of RAM or more. That rules out the entry level models like the hugely popular Nokia Lumia 520, at least for now. There is talk that the Lockscreen will eventually be available for models sporting less than 1GB of RAM.
Belfiore's tweet also mentioned that the developers ran into some trouble while building the app, and also added a new feature. If Joe's timetable is correct, by this time next week, the beta Lockscreen app for Windows Phone 8.1 should be ready to be downloaded.
source: @joebelfiore via WPCentral