The iPad 3 GPU is a beast compared to Nvidia Tegra 3 and Qualcomm S4
Just to put the power of the A5x GPU in perspective, I went and looked up the maximum resolution for its 2 biggests contenders: The Nvidia Tegra 3 and the Qualcomm S4.
What I found is that the A5X is pretty damn remarkable.
Apple A5X GPU Max Reso: 2048-by-1536
Top Qualcomm S4 GPU Max Reso: 1900-by-1200
Nvidia Tegra 3 max reso: 1680-by-1050
So right now, even if Android and Windows OEM's were able to score a Retina display, they wouldn't even be able to find a ARM powered GPU to power it.
That shows how far ahead of the competition Apple is and shows why being in control of your own production line and CPU/GPU is so valuable. They are not limited by what current GPU can handles, they are limited only by their imagination