Which versions of Xcode are not updated to take advantage of HiDPI/Retina displays?
The current non-beta version of Xcode, 4.3.3 for Lion, works fine on a retina device but certain UI elements are not retina-enabled. Most notably the little "LCD" screen that sits in the top middle of the toolbar, it looks quite blurry on a retina MBP. Also the Play/Stop buttons, and many other UI elements. 

However the text used to render your code is crisp.