GDR3 is coming to Windows Phone 8 soon.
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.
This should make it really easy to support such devices. You just design and develop for them in the same way as for 720P (and everything else-probably) and let the autoscaler take care of everything else.
I expect that the SDK will also be updated to add appropriate emulator and designer support too.
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 future new screen size?
Nothing yet, just be prepared to possibly add larger sized images-if you want/need them.
Plus expect to have to test on more devices in future too.