I was going to ask about this.
It turns out that this can be tested on current HW. You just need to use QuartzDebug to enable HiDPI mode. Once you do that, you can select a HiDPI setting in the System Prefs > Display Control panel; that will simulate the retina display functionality (everything will look huge, but it shows how the software operates). This is on Lion or newer, of course.
When I do this with our JUCE based plugin using, say, AU Lab, what I see is that AU Lab window is rendered in Retina mode (e.g. the title bar, etc.), but that the JUCE content is upscaled. This test was just done without CoreGraphics rendering due to the conflict between CG in Juce and AAX. It may be that using CG rendering will have an impact; I will try that and report back -- but it is not clear that CG is compatible with audio processing as currently implemented.
In any case, it would be good to be able support Juce rendering for Retina targets; this would appear to imply some translation between the NSView implementation and the component layer; I was wondering if anyone was planning on digging into that?