I broke down and bought myself a new retina Mac Book Pro a while ago. This is an amazing machine and that display is absolutely stunning. One thing that I thought is that Lightroom needed an update to support the retina display (Aperture and iPhoto already support it and they look fantastic) so I wasn't expecting that much from Lightroom. However after browsing for a while through my library I started noticing that the Library previews were really sharp. Taking a screenshot and blowing it up confirms that Library actually is using the full retina resolution, regardless of what setting you use for the display scaling. That is superb. Unfortunately Develop doesn't yet know about the retina display so there you still get the low resolution which makes for a big difference in feel for the image. I did notice that it is probably a good idea to turn up the size of your standard previews to get the full benefit. Here is the proof using 1:1 screenshots.
Left: Library fit view. Right: Develop fit view.
Ironically you will see the difference between these two better on a non-retina display as the browser doesn't automatically scale 1:1 but you get the gist.
Update: for retina owners, here the same screenshots at what should be 1:1 if you use "best for retina"