Simply put, we need to define what a Retina Display is on the iPad.
The Retina Display, in its current form is a display that has a higher amount of pixels per inch that can’t be discerned by the human eye. Our eye’s limits is 300 PPI which the iPhone and iPod Touch beat by 23 pixels. A Retina Display on either of these iOS devices makes sense as the hardware can support the resolution (960 x 480) and context exists within this resolution.
The jump to the iPhone 4’s Retina Display resolution is a doubling of vertical and horizontal pixels, or quadruple the number of pixels across the 3.5″ display. Pixel doubling on the iPad to achieve a retina display means developing a 2048 by 1536 pixel display. To put that in perspective, I recommend reading John Gruber’s post Cold Water on the iPad 2 Retina Display Hype which further explains the Retina Display hype on the iPad and why it might not be happening anytime soon.