The target audience for the iPad comes nowhere near technology rumors sites. They don't know what a Retina display is, probably haven't even heard the expression before and accordingly don't care.
The iPad is a device for people who don't care about technology. They want something that makes surfing the web and consuming content easy for them. They don't care if the damn thing has one, two or 4,096 CPU cores and they don't care about how many trillion pixels the display has -- as long as they can hit YouTube and iTunes with it.
So, NO, the availability of a Retina display will not effect iPad sales at all. The fistful of people that always want the latest and greatest and shiniest don't really matter in the big picture. Remember? The MacBook was the most successful Apple notebook, not the high-end 17" MacBook Pro and the iMac always out-sold the Mac Pro (or PowerMac) by far.
Besides, we've also heard the rumors that this Retina Display iPad might probably be named "iPad Pro" -- which means that there might be TWO different iPad product lines in the future. The one with the current display resolution and the one with the high resolution display. But one thing should be absolutely clear: Unless they subsidize it, Apple cannot sell a Retina display iPad at the price of the current iPad. The added costs of the high resolution display alone will be prohibitive.
I'd expect the low-end Retina iPad 3 (16GB without 3G) to cost at least as much as the high-end iPad 2 (64GB with 3G). I mean, I'd love to see the Retina version to replace the current product line at the same price level, but let's be realistic here: That's just not possible.