Microsoft has yet to release any information about Windows Phone 8.1. That includes any details on a potential preview program for developers and/or early adopters. However, Microsoft has, in the past, indicated that the Preview for Developers program would continue for "future versions of the platforms".
Sources close to Microsoft have indicated that the updated Preview for Developers app is one step in a program that will deliver Windows Phone 8.1 to interested parties ahead of general retail availability.
The update from Windows Phone 8 to Windows Phone 8.1 is a significantly bigger step that from Windows Phone 8 Update 2 to Windows Phone Update 3 (i.e. major update versus minor update). Windows Phone 8.1 is expected to include both new features (e.g. notification centre), updates to existing features (e.g. improved voice recognition features), and a large number of API additions and updates (e.g. new API for Bluetooth Low Energy).
This does make the update process more complex (testing, compatibility, etc.), but it is still being delivered as an in place over the air update. However, there are some suggestion that this may mean that not all devices will be able to be access / be part of the Preview for Developers program (though they will be updated later). Do note that none of this information has been officially confirmed, as ahead of any formal announcement there is no formal acknowledgement that Windows Phone 8.1 even exists.
Windows Phone Store description:
Now registered Windows Phone developers can receive pre-release OS updates on their dev phone, directly from Microsoft! You can use this app to opt-in your phone to receive early Windows Phone updates before they are generally available, enabling you to run and test your app on real hardware running the latest OS before your customers.
- Access to latest pre-release Windows Phone updates
- Ability to opt-out of pre-releases and only receive production Windows Phone updates
The Preview for Developers app can be downloaded from the Windows Phone Store