Retina iPads will happen...someday
I believe they will have retina displays on the iPad in the future but not today.
There's too many device fragmentation considering the graphical resources will have to be bumped up to 2048x1536 of all existing iPad apps and the just iPad was released a few months ago.
There will be a time of a "display transision" like how the iPhone 4 has transitioned from 480x320 to 960x640. There are still a lot of iPhone apps that haven't been updated to match iPhone 4 resolution. It will take time.
The PowerVR GPU that exists on all iOS devices currently has to be able to push and maintain pixel performance at that resolution. They probably have prototypes that won't be released 2-3 years later.
You know how Apple releases products? They ROLL. Release new models with a few core feature at a time and make it great, no rushing unlike other companies who put everything at once.
Just recap with the iPhone's hardware changes:
1G - initial release
2G - Added 3G speed, GPS
3G - "3GS" Speed and RAM upgrade, Compass, 3.2MP camera.
4G - "iPhone 4" Retina Display, Facetime, Gyroscope, Improved Rear Camera, Wifi N
iPad just had its initial release. iPad 2G with a camera make sense, NOW they have FaceTime implemented. A 7" inch iPad is a possibility, now that you can have a similar but not quite Retina Display by shrinking pixels of the same resolution. iPad 3G could have Retina Displays, who knows....