Apparently there's something called GDR3 coming to Windows Phone 8 at some point in the future. Allegedly it will include support for devices with 1080p screens.
As developers, is there anything we can do to be prepared for working with such devices?
1080p is 1.5 times larger (in both dimensions) than 720p.
I expect that the SDK will also be updated to add appropriate emulator and designer support.
If you want or need to use different images (or other resources) at different sizes, you can handle this in the same way as you would at the moment. (MSDN article explaining how.)
So what can or should developers do now to be prepared for a new screen size in the future?
Nothing yet, just be prepared to possibly add larger images if you want or need them. Expect to have to test on more devices in the future too.