Might Apple update the MacBook Pro with a high pixel density/DPI/resolution "Retina Display"?

One other aspect that hasn't yet been mentioned here is the price. There was speculation that the iPad 2 might get a retina display or might not because the manufacturing costs would be too high. For a 13"/15"/17" screen those costs would be much higher. I think we will have retina displays in iPad 2 or iPad 3 before we see it coming to MacBooks (because of all those OS X issues others mentioned here). On smaller screens like iPhone and iPad it makes more sense. Actually, the current resolution of the iPad is quite bad at 132 pixels per inch - as opposed to the 169 pixels per inch of the Android Samsung Galaxy Tab or the 326 pixels per inch of the iPhone 4.

http://www.apple.com/macbook-pro/ The MacBook Pro with Retina Display.

No, because the OS X operating system and applications do not support resolution independence.  Resolution independence is a technology where graphics are stored as vector graphics rather than bitmaps, and can be scaled by the OS to adjust to any pixel density on the screen.  If OS X is running on a screen with a very high resolution, everything is going to be really tiny unless all of the interface elements in the OS, and all of the applications, use vector graphics so they can be scaled up to a usable size.  To accomplish this, Apple would have to coordinate with thousands of application vendors to redo their applications to use vector graphics, so they would work at the higher resolution.

You can get 15" WUXGA laptop screens that are 1920x1200, I believe that is the highest resolution presently. A limiting factor is resolution independence in OS X. It is only partially supported presently.

Yes, but it will require a major OS update that incorporates the same high-dpi display support that Apple added to the iPhone 4. Fortunately, Apple has announced that the key theme of the next major OS X update (Lion) is incorporating advancements from iOS into the desktop OS. I would guess it is quite likely that high-dpi support will be one of these. The other hardware factors limiting a MacBook Retina display (display fabrication, powerful enough GPU) they are probably tackling with the iPad first. Even if these were available now for the MacBook, the biggest limiting factor for it is OS X support.