I may be way off the mark here but this comes from my experience with PlayN (which seems very similar to libGDX).
Though Retina means double resolution you treat it as if the resolution were the same as a non-retina device. Apples core libraries will even tell you that the screen if half the size as it actually is!
When you load an image asset, say "Sprites.png", iOS will actually first look for a file called "Sprites@2x.png" and use that file instead.
When coding for iOS you're are no longer working in pixels - but in points - and its up to iOS how big a point is depending on the context.
So all you need to do is provide versions of all of your image assets at twice the resolution with a slightly different file name e.g. "Sprites.png" and "Sprites@2x.png". The iOS will do the rest :D
I hope that makes sense... I didn't get much sleep last night :)