Methinks you are a bit confused as to what "resolution" means. Here are the screen resolutions of various things:
iPhone / iPhone 3G / iPhone 3GS = 480 x 320
iPhone 4 = 960 x 640 (exactly double that of the original iPhone)
iPad / iPad 2 = 1024 x 768
current MacBooks / MacBook Pros = 1280 x 800, 1366 x 768, and up
So iPhone < iPhone 4 < iPad < MacBook/MacBook Pro when it comes to resolution.
The OTHER thing to measure is DPI -- Dots Per Inch, or pixels per inch. In that area, the iPhone 4 is superior to the others because it packs more pixels into each square inch than any other Apple display. If the iPad had the same DPI as an iPhone 4, the resolution would be comparable to a 30" Apple Cinema Display -- and that's MASSIVE resolution. The main reason for getting the "Retina Display" on the iPhone 4 first is that it has a comparably small screen.
When you view a "Retina Display" at an appropriate distance, you don't really see the pixels anymore -- they are individually smaller than the human eye can discern. For that reason, it seems self-defeating for DPI levels to get much higher than "Retina Display" level. There will come a point when resolutions stop getting higher and displays just start getting cheaper and better (e.g., better, more accurate display of colors, etc.).