Currently, Photoshop CS6 is not optimised for the Retina display, so things will appear very similar to the way they are on a non-Retina display. I believe this means each pixel is displayed in your document as a 2×2 pixel block, as is the preview in Save For Web. This is the same as how non-Retina apps are displayed on a Retina iPhone.
Basically, Photoshop is identical on a Retina Mac as it was previously. It'll be like this until Adobe update Photoshop. It's pixel accurate, but each pixel is drawn with 4 Retina pixels (a 2×2 pixel block). It'll be just as easy as it was previously to work with, but for Photoshop itself, there won't be much benefit (just higher res text in menus etc).
Adobe hasn't announced when the Photoshop Retina update is coming, or if it'll be CS6, CS6.5 or CS7.
24bit IPS display
The new Retina MacBook Pro does feature a 24bit IPS display. This is a huge advantage over the older 18bit displays on previous MacBooks, MacBook Airs, MacBook Pros and even some iMacs. 24bit IPS displays are pretty rare for laptops.
Other resolutions and scaling
There is a very big caveat when using a Retina MacBook Pro — you'll want to make sure you're using the native size of 2880×1800 pixels (1440×900 points). This is called “Best for Retina Display” in System Preferences.
If you switch to one of the other display scales, everything gets rendered at double size then scaled for the screen. So 1920×1200 is rendered at 3840×2400 off screen, then 2D bitmap scaled as a single texture to 2880×1800. In that situation, what you're seeing will no longer be pixel accurate. For some situations that will be acceptable, but for UI and icon design I'd advise against it. This is the “Scaled” mode in System Preferences.