One of the more intriguing Windows Phone 8.1 revelations to come out of Build a few months back was that users would be allowed to customize their handsets' lock screen via new interactive apps provided by Microsoft and, eventually, third parties. But this lock screen capability was not provided to testers, and it is not part of the RTM version of Windows Phone 8.1. What's up?

To be clear, there are three components to this functionality. First is a set of APIs that enable developers to create new interactive lock screen experiences that completely change how the lock screen looks and works. Second is a Microsoft lock screen app that will provide a set of new lock screen experiences. And third is the opening up of these capabilities to third party developers so that they can also provide users with custom lock screen experiences.

Thanks to a couple of tweets by Microsoft corporate vice president of Windows Phone Program Management Joe Belfiore, we now know a bit more. That is, we have a rough timeline for the delivery of Microsoft's lock screen app.

"[The] new lock screen app [is] probably a couple weeks [away] as [a] beta for 1 GB devices," one tweet notes. "A bunch of us [at Microsoft] are running it now, working on polish. Dev[eloper] preview [is] next."

A second tweet clarifies the situation for lower-end devices with just 512 MB of RAM.

"Why not 512 [MB handsets]?" he wrote. "To make lock fast/stay in memory. 512 MB for lock app will probably be a while."

If you were paying attention to the Build announcements, you may recall that this lock screen customization capability seems like a big deal. In my Windows Phone 8.1 Preview, I noted that extensibility was a bit theme in this release, and that the lock screen apps would "provide custom visuals and animations. The few they demonstrated at Build were quite attractive."

Here's how Joe explained the feature at Build.

"In Windows Phone 8.1, we've implemented a new set of APIs for the lock screen, which we're using ourselves for a little while, before we publish them all to [developers]," he said. "These APIs enable lock screen experiences that are highly interactive and unique, where the application can take over full control and really render whatever it would like."

"As the first app to use these [APIs], we've been collaborating with star Windows Phone developer Rudy Huyn. He's been working with us to build a first experience which we, Microsoft, are going to publish in the [Windows Phone] Store in the coming months to give users a wide range of highly varied lock screen themes."

"These lock screen experiences can be highly varied," he said. "The sky is the limit in terms of what [developers] will be able to do with this creativity once it's available."

He showed off a few of these themes, and each offers some cool animations that are not demonstrated in still shots. This one is called Diagonal.

And this is called Tokyo.

So there you go. The APIs are complete. Microsoft's lock screen app will appear in beta soon, but only for devices with 1 GB or more of RAM. Support will possibly be added for 512 MB devices. And third party lock screen apps? Sometime (still unknown) in the future.