One of the iPad mini's chief criticisms--if not its only one--is its lack of a retina display. To many of who own a post-3GS iPhone or post-third generation iPod touch, new or newer iPad, or one of the two top-of-the-line MacBook Pro models, the difference is immediately noticeable. No matter how stunning the iPad mini is, the 1024x768 screen is a noticeable drawback.

How noticeable, exactly? In geek terms, about 150 pixels per inch. Take a look at the pixel density for each of Apple's iOS products:

326 pixels per inch (iPhone, iPod touch)
264 pixels per inch (iPad)
163 pixels per inch (iPad mini)
132 pixels per inch (iPad 2)

During the iPhone 4 keynote, when "retina" was introduced into our vocabularies, Steve Jobs defined the parameters for such a display: "It turns out there’s a magic number right around 300 pixels per inch, that when you hold something around 10 to 12 inches away from your eyes, is the limit of the human retina to differentiate the pixels." This is why the iPhone and iPod touch have more pixels per inch than the iPad; naturally you hold those devices closer to your eyes.

Despite being the same resolution, the iPad mini also has a higher pixel density than the iPad 2, due to the smaller screen; therefore, a retina iPad mini would have a significantly higher PPI than the iPad. Calculating it based on the current design (using the doubled resolution of the new iPad display and the PPI formula*) yields an iPhone-like pixel density of 326.

That's higher than the iPad's 264, of course, but based on Steve's fairly unscientific mantra, it should be, since you'll be naturally holding the iPad mini closer to your face. Besides, Apple's certainly not going to create a new resolution just to give the larger iPad PPI bragging rights; nor is it going to deprive iPad mini users of the joys of a retina display for too long.

But I don't necessarily think the next mini revision will bring retina across the board. Rather, I think there will be a premium attached to it, much like the MacBook Pros. Note the new branding of the iPad. Instead of the "new iPad," like with the third generation, Apple has adopted its MacBook language, calling it the "iPad with Retina display."

I think we'll see the same thing with the mini, with a price cut for the updated-but-still-non-retina model (likely to $299), and a new iPad mini with Retina display sliding into the high-end model.

The mini is nearly sold out, and the iPad 2 is still a big seller for Apple. But if retina matters to you, it shouldn't be too long before you're holding a mini as close to your eyes as you want.

*To determine PPI, the formula is as follow: (Diagonal resolution {Square root of [(Resolution width squared) + (resolution length squared)]} divided by diagonal screen size). In the case of a retina iPad mini, it would be the square root of (2048 x 2048) + (1536 x 1536)/7.85, or 2560/7.85.

Find Michael Simon on Twitter or App.net as @morlium.