It's fairly safe to assume that any app that works with Windows Phone 8 at the moment will work with Windows Phone 8.1 when it is released, or at the very least will be updated to add compatibility. But if you were in any doubt about whether your favorite app will work with the successor to Windows Phone 8, the Windows Phone Store will let you know.
This is handy to know, but more importantly it also reveals what we have suspected for a long time -- that the follow up to Windows Phone 8 will indeed be called Windows Phone 8.1. There has still been no official announcement about the name or release date -- we should find out more at BUILD next month -- but the information in the Windows Phone Store means that the name is now all but certain.
Take a look at the listing for Facebook Messenger for example. Cast your eyes over to the "Works with" section of the app's details, and you'll see that both Windows Phone 8 and Windows Phone 8.1 are listed.
So unless something fairly drastic happens in the coming weeks, any debate about what the next version of Windows Phone is going to be called can come to an end. It may not have been confirmed by Microsoft (yet), but it looks like the rumors (and what common sense dictated) were true. Windows Phone 8.1 is the next big mobile release from Microsoft.