A 2048 × 1536 iPad display would seemingly be cost prohibitive today. Not just for the display itself, but for the RAM. The current iPad has 256 MB of RAM, which is shared between the CPU and GPU. I don’t think 512 MB of RAM would be enough for an iPad with a 2048 × 1536 display.1 That’s almost as many total pixels as on a 27-inch Cinema Display (resolution: 2560 × 1440).
He also writes in a foot note:
Note, for example, that the 27-inch iMac ships with a graphics card with 512 MB of RAM. Even the MacBook Air has 256 MB of RAM on its graphics card.
He is discussing whether the iPad 2 may or may not have a "Retina Display" which would be twice as large as todays 1024x768 display. He postulates that twice the resolution (2048x1536) would be a large strain on the video RAM in the iPad, which currently has 256 shared RAM. Well, if we ignore the possibility of iPad 2 having more shared RAM, the actual resolution of the screen doesn't require all that memory.
Basically, when calculating VRAM, you take the dip depth times the amount of pixels on screen. You should probably double the result for frame buffering.
So the math is fairly simple: (32 * 2048 * 1536) / 8 = 12582912, or almost 13MB of VRAM, just to address these pixels. Double that and you end up at 26MB of VRAM for the screen.
The 512MB of VRAM in modern Macs are not there to be able to address all those pixels, but rather to handle advanced 3D algorithms for games and 3D applications. The more memory in the GFX card, the more data can be handled at any given point which speeds up the game or application.
Now, the disclaimer here is that even though the display in itself only needs 26MB of RAM, the iPad does indeed have OpenGL support and there are tons of popular (and amazing) 3D games available for it, so using all those pixels to display a complex 3D environment would indeed requires a lot of memory (that the iPad2 very well could have) and a very good graphics card. These are generally cards that cost something north of $500 and require a power brick all to themselves. That is more likely a good reason why the iPad won't be twice the resolution.
That said, It's quite possible to not run games in 1:1 on that Retina Display, and give OpenGL only access to a 1024x768 pixel space and then double it for the screen.
iBooks would look awesome, nonetheless.