I haven't used a Mac with a retina display yet, but I have noticed that when I went from an iPhone 3GS (non-retina) to a 4 (retina), suddenly tons of websites that I previously had to zoom to read were easily readable as-is — lots of text that had been literally illegible at "100%" was now pretty readable and text that was bigger but still small enough to be fuzzy was now crystal clear.

I don't know how much that will translate over to a computer, but it wouldn't surprise me if people who work with text a lot (including programmers) are able to comfortably use significantly smaller font sizes than they ever were able to before.