[timr]

Yeah, maybe...but there's still a practical limit. In order to see the pixels on a thunderbolt display (again, 109ppi) I have to get my face about 7 inches away from the screen.

Closer than about 5 inches, and I lose the ability to focus because the screen is too close -- so there's a band of about 2 inches where I can gain from a higher pixel density than 109ppi, without losing due to eyestrain. And in any case, I'm not going to spend much time in that zone. It's hard to work with your nose in the screen.

YMMV, but I think I'm fairly typical. Most people dramatically overestimate the precision of their eyes.

[rayiner]

You can see image degradation from pixelation long before you can make out individual pixels. I can't really make out individual pixels on my MBA (130 ppi) at one foot, but looking at a MBP Retina at the same distance looks dramatically better. On the MBA, the fuzziness from the heavy anti-aliasing used to hide the pixelation is quite apparent, but on the MBP Retina pixels look like sharp-edged solid shapes.

[photojosh]

I don't know whether it's due solely to the resolution, but I was pretty shocked to realise I can tell the difference between 300 and 600 dpi photographic prints (assuming there's enough detail in the image to do so, you need to print a 23 MP DSLR shot with high detail at a 7x10" print size to get there). I have had other photographers tell me that they don't see any benefit to retina screens at all... you're giving me the reason why here. :)