UPDATE: As of December 2012, Adobe has made updates to their software to now properly support Retina Displays.
I had some time to check out a new MacBook Pro with Retina Display at
the local Apple Store. I was very impressed with the display, but I
needed to see how it would look when using my usual applications.
They didn't have Lightroom installed, so I experimented with Photoshop
CS5 instead. I opened a full-resolution JPG in Preview and then
opened the same photo in CS5. I compared how the CS5 (non-Retina)
looked to the Preview (Retina) version in the background. The Preview
version was a 100% crop and the CS5 version was viewed at 50% to match
scale. The difference in fine detail is very noticeable. Have a look
for yourself at the full resolution version of this upload. In my opinion, it will not be possible to do detailed photo editing
such as sharpening on a Retina display until Adobe updates its
products for Retina graphics.
Photoshop and other non-Retina-capable applications think they're working with a 1440x900 display, so that is the graphic the program is generating. Meanwhile, the Mac knows it needs to scale all that up to 2880x1800. This is done by taking each pixel and expanding it to a four pixel area on screen (called pixel-doubling, since each linear dimension is doubled). That's why it looks pixelated.
In addition to the Adobe test, I also tried setting the display to different scaled resolutions and I was blown away at how good everything still looks (for applications that have Retina support, of course). The scaled 1920x1200 option appears just as sharp as the native 1920x1200 resolution on my current 17" MacBook Pro. Way to go, Apple! Unfortunately, non-Retina applications still look bad at the alternative resolutions.
Once the 3rd-party software catches up, this will be an amazing piece of hardware.
Apple Fanboys, please don't hate - I'm one of you.