Well, although Retina displays at 2880 x 1800, that doesn't mean your movies need to be that resolution. In fact, there's no consumer format that's near that resolution! (Even BluRay is only 1920x1080 -- and that can be shown on a 50 inch or larger TV screen without any resolution problems!)

Have you looked at a 1280x720 video on your iPad? Chances are that Retina uses software to make it appear at 2880x1800. If that's not good enough, you can , I suppose, go as high as 1920x1080. But you may not see a dramatically improved picture on your 10 inch screen. Likewise, rendering your video at a slightly reduced quality may not visually affect your playback on your iPad at all -- although it produces a smaller, more efficient file.

Remember that portable video devices must balance between quality and portability of video. So dumping more video data into your video may work your iPad harder with no discernible improvement in quality.

That said, would you mind doing an experiment for us and reporting back? Try outputting a good, quality video at a Render As format for iPad (Main Concept AVC Apple iPad/iPhone 4 720p30) and then output the same video as, say, a Main Concept AVC Sony Tablet 1080p and see if you can see a significant difference on your iPad. (You can also try a BluRay output and see if it's worth the extra file size.)

Your results could help someone in the future.