Fueled by speculation from smart and generally well-informed blogger John Gruber, claims circulated over the weekend that the second-generation iPad will have a screen with quadruple the pixels of the original iPad—2048 by 1536 (QXGA) resolution. Implied to be a guess (“my money is on”) but wisely hedged at the end (“I’ll believe it when I see it”), Gruber’s speculation appears to be based on the following logic:

* Apple quadrupled the pixels of the original 480×320 iPhone and iPod touch screens in order to achieve Retina Display resolutions in their respective fourth-generation models.
* Quadrupling pixels (doubling them in both horizontal and vertical directions) offers the most straightforward option for upgrading a screen while preserving compatibility of old software.
* Apple knows that higher-resolution displays make its portable devices more attractive to consumers and more difficult for rivals to match on specs.

Each of these statements is demonstrably true, and two additional factors helped the guess to pick up steam:

* Recent reports have suggested (again) that Apple will be using a more powerful PowerVR SGX543 graphics processor and multi-core CPU in the new iPad.
* Discovery of some higher-resolution “iPad X2” art tiles in iBooks, suggesting that Apple had designed some of the graphics for a quadruple-resolution display.

So that’s it, right? An ultra-high resolution screen in an iPad would make sense, given Apple’s desire to offer products its rivals can’t match, and if the second iPad has improved CPU/GPU hardware, a new display should be a lock. Well, not really; it’s worth considering the other side of the equation.

History has shown that Apple very rarely makes screen resolution changes, let alone radical ones, in its second-generation products. No matter what else it may change under the hood, it generally picks a target resolution for a first-generation product’s screen and sticks with it for at least one or two more years, as was demonstrated by every iPod model, the first three iPhones, and most of its Mac computers. Though it’s not a spec-obsessed company, it tends not to step on the toes of a more expensive product line by offering hugely better features in a lower-priced device. A 2048 by 1536 iPad at $499 (or even $599) would blow away the displays on every Mac computer Apple makes, except—arguably—a $1,699 27” iMac or a Mac with the $999 27” LED Cinema Display. That list includes:

* The standard 13” MacBook. At 1280 by 800, it offers a resolution close to the current iPad, and hasn’t changed in years—its iBook predecessor had a 13” 1024×768 screen for years.

* Every MacBook Air. After a generation or two of matching the MacBook, the 13” Air just gained a resolution bump to 1440 by 900. Even the new 11” Air surpasses the standard MacBook at 1366 by 768. These are small differences, suggesting how little Apple is willing (or able) to boost the screen specs on its thinnest machines.

* Every MacBook Pro. The $2,299 17” version tops out at 1920 by 1200, or slightly higher than 1080p full HD resolution; this is still lower than the proposed iPad 2 display. Meanwhile, the 13” Pro is currently equivalent in resolution to the standard 13” MacBook.

* The 21” iMac. This model falls a little short of the 17” MacBook Pro by offering 1920 by 1080 pixels, equivalent to full 1080p high-definition resolution. It starts at $1,119.

* Almost any other Mac with a Cinema Display. Although the current 27” LED Cinema Display has a 2560 by 1440 screen—higher than the supposed iPad 2 screen on one axis, lower on the other—the prior 24” version had 1920 by 1200 resolution; only the discontinued 30” version was higher at 2560 by 1600.

In other words, a 2048 by 1536 second-generation iPad screen would not only be roughly on par with what’s in Apple’s most expensive computers and monitors, but it would also have to fit all those pixels into a roughly 10” diagonal display—a display that most likely doesn’t exist. A quick check of LCD screen maker Samsung’s website suggests that its 9.7” displays tap out at 1024×768, the iPad’s current resolution, and other reported iPad screen suppliers LG and Chimei Innolux don’t appear to sell sub-10” screens with anywhere near the pixels discussed above; the iPad’s screen is closer to the high end than the middle or bottom of its product class. Apple would need screens that it could reliably source in the tens of millions (reportedly 65-million) per year, so unless it has had secret factories working on QXGA iPad displays for a couple of years, finding such parts would be unlikely. Additionally, even if Apple did in fact include a supercharged video processor to power a super screen, the iPad’s notebook-besting battery life could be impacted considerably when apps demand four times the pixel changes of the prior iPad. It’s far more likely that a display similar to the current iPad’s would be given the optional ability to display better 3-D and 2-D graphics than an outright mandate to do so.

It’s worth considering that upcoming iPad rivals aren’t targeting ultra high-resolution displays, either. RIM’s PlayBook has a 1024×600 screen at 7”, and Motorola’s Xoom has a 1280×800 screen at 10.1”, which will likely be similar or superior to most of the Android 3.0 tablets offered by other companies for most of this year. These products will very likely use similar CPU and GPU technology to what Apple is expected to be including in the second-generation iPad. In this context, it’s easier to give greater weight to other discoveries of new iPad features suggesting that the 1024×768 resolution will stay the same, perhaps benefitting from improved backlighting, color gamut, or other non-resolution changes.

None of this is to say that an ultra high-resolution display for the iPad is impossible in 2011. Apple would surely love to release an iPad with DPI akin to printed documents. It’s just that such a huge jump in resolution seems really unlikely, particularly in a second-generation Apple product. The original iPod stayed with a 160 by 128 screen for four generations before receiving a minor boost and going color; the iPod nano spent two generations at 176×132 before rivaling the iPod at 320×240 in the third. High-resolution displays weren’t anywhere near Apple’s list of priorities until eight years had passed, and the company repeatedly went with small resolution tweaks rather than the big ones people dreamed about every year. A 2048 by 1536 display would instantly make the iPad 2 not only HD-ready, but superior to most of the HDTVs sold today. As much as we’d love for this to happen, it seems next to impossible. So keep your fingers crossed, but don’t put your money on it as anything other than a long shot.