Apple is set to reveal the successor of the first iPad at an event to be held in San Francisco (either the company's HQ or Yerba Buena Center for Arts) by the end of this month; one thing is sure though, the iPad is unlikely to feature the iPhone 4's Retina Display.
With a resolution of 960x640, the iPhone 4 has the highest screen resolution of any mobile phone and the highest pixel density at 324 ppi; but this comes at a price; $28.50 or roughly a sixth of the estimated manufacturing price (bill of material) of the handset.
In comparison, the current iPad has a screen resolution of 1024x768 pixels spread over 9.7-inch, this translates into a density of only 132 ppi which is similar to the MacBook Air; the touchscreen display carries an estimated cost price of $95 or roughly the a third of the price of the least expensive Apple iPad.
Going for an even higher PPI is likely to push the price of the screen upwards, something which is not sustainable if Apple wants to maintain its profit margin without increasing the retail price. Sticking to the same screen makes sense as it would help the manufacturers (and developers) amortise their investment in tools.
Then there's the fact that a Retina Display would push the screen resolution, on an iPad-size display to a whopping 2560x1920 which is more than twice the amount of pixels displayed by a full HD device.
As it stands, this would max out even the most powerful mobile graphics hardware on the market, making it a seemingly impossible challenge for graphics and battery engineers; more pixels equates to more transistors and even higher power consumption.
Developers will also need to take care of upscaling applications so that they can work smoothly on a display that can display more than five times the screen resolution of other smartphones.
It would be interesting however to see if Apple chooses to release a HD-compliant version of the iPad, moving to 1280x720 pixels and finally bringing the 4:3 screen format to the iPad.