to run a game on a retina device, you can set it's "requestedDisplayResolution" to "high"; that way, it will run in HD resolution on a retina iPhone and in half that resolution on a non-retina device.
The problem is that the HD textures will likely be too expensive to render on a non-retina device.
On iOS, this is normally solved by using 2 sets of textures: normal textures and HD textures. You always create the game in a coordinate system of 480x320 (low resolution), and at run-time, different textures will be loaded depending on the device.
The problem is that both sets of textures always have to return the SD "width" and "height", so that your images are displayed at the correct level.
Starling does not support this currently, but you could easily add this manually for now, by adding a "scaleFactor" property to your textures, and using that factor in the "width" and "height" properties. Then, depending on the device you're running, use different textures and set the "scaleFactor" accordingly.
I know this is a rather rough how-to, but perhaps it gets you going! I plan to write a tutorial on that when I implement that feature -- but it might take me a while.