Update scheduling and timeline
GDR2 was RTM (released to manufacturers) in the late spring, but it takes some time for the software to become available on devices. This is due to testing by both manufacturers and operators.
The Nokia Lumia 925 was the first Windows Phone device to get GDR2, marking the beginning of the roll out of Windows Phone's latest software update. This is typical for newly announced / launched devices, which are generally tested and released with the latest version of the software earlier than existing devices (understandably they receive priority treatment).
The software is then tested on and released for existing devices. Sometimes, as with GDR2, it is newly manufactured devices that get the update first. This is because, once the software update becomes available for an existing device, the time to go from factory to shop shelves is generally slightly less than the time need to test and deliver the over air the air update process. The timing of this can depend on stock levels, as only in rare cases will existing stock be updated to the new software (usually requiring a return to the distributor or manufacturer).
GDR2 now available on newly manufactured Samsung devices
Currently, it appears that it is only newly manufactured Samsung devices that are shipping with GDR2. However, we would expect existing newly manufactured HTC and Nokia devices to follow in due course. Because of the generally higher stock level of Nokia and HTC Windows Phone devices in retail channels (a consequence of their higher sales levels), we would anticipate that newly manufactured Nokia and HTC devices will receive the update at roughly the same time that the over the air update is released for the same devices.
The over the air software updates for GDR2 are scheduled to begin this week, but the process will be gradual. The majority of Windows Phone 8 devices are expected to receive the update before the end of August, but there will inevitably be exceptions as operators drag their heels over testing and certification.
Windows Phone GDR2 (version 8.0.10327.77)
We've recently posted a number of features looking at some of these new features in GDR2, notably on the return of FM Radio, the wider availability of Data Sense, and the addition of CardDAV / CalDAV (Google) support.
Microsoft official change for GDR2 (build 10327 or 10328) is also helpful, listing the following updates:
Xbox Music. It's now easier to select, download, and pin music. Plus, music fans will see more accurate metadata (such as song and album information) and many other performance improvements.
FM radio. Listen to FM radio right from the Music+Videos hub. (Not available for all phones.)
Data Sense. Stay on top of your phone's data usage with Data Sense. You can set a limit based on your data plan, or just keep track of which apps use the most data. (Not all mobile operators offer Data Sense.)
Skype. Voice over Internet Protocol (VoIP) apps like Lync and Skype now feature improved stability and performance.
Internet Explorer. Your web browsing experience is now better than ever with improved HTML 5 audio playback.
Camera. Set your favorite Lens so it opens automatically when you press the camera button. (Not available for all phones.)
Other improvements. Includes many other improvements to Windows Phone.
Addendum
WParea.de reports that existing SIM-free Samsung ATIV S devices are receiving the GDR2 update over the air. It looks like Samsung is first out of the gate with update, but we expect other manufacturers to follow shortly.
Thanks to PL and ET for the information on Samsung devices.