@braitsch - I believe that was a change that was made intentionally as a start towards getting retina better supported in OS X. On iOS this is how things behave - (if you enable retina you get a window with twice as many pixels).
The reason it is different between 0.9.0 and 0.9.1 is that in 0.9.0 the screen width was actually not getting reported correctly.
as an experiment if you try drawing a circle at (1024, 0) in 0.9.0 and in 0.9.1 they should both draw in the top middle of the screen as the screen size isn't different between 0.9 and 0.9.1 but the width was not being reported correctly for retina enabled apps.
I hope that explains it.
Right now for retina you have to handle the scaling etc yourself - (we just give you a bigger frame buffer)
For 0.10.0 we should aim to have the ability to have the retina scale passed through to the renderer so it requires no changes on the user's end.