A WPC forum member named xs2k claims to have bought a developer edition Lumia 920 running GDR3 and Lumia Bittersweet Shimmer update (earlier reported by one of our tipsters) has posted many screenshots from the device. Following changes can be seen in above collection of screenshots.
Scren Rotation lock
New multitasking screen with ability to close apps
OS version: 8.0.10484.106
Apart from the above changes a major change is that,
you can now sign into wifi when setting up your phone. That means you can restore your phone if you don’t have any data connection.
Though the user is not able to confirm separate volume levels for various functions as has been rumored coming with GDR3.
You can see more screenshots at WPC forum.