While you can't calibrate an iPad, it is possible to profile one. We have quite recently been creating profiles for personal display devices like phones for some clients. No iPad profile yet. It's not easy and involves several different hardware and software combinations as well as custom web pages we've put together ourselves. But once you have a profile, you can convert your images to that profile and put the image into "iphone space" or "iPad space" so that your image looks correct on that display.
Keep in mind that the color of the backlight of an iPad is not adjustable as are other monitors. My guess is it would be somewhere around 7100 K - a bit blue. And the rest of the image is scaled toward the white point. The whites on the iPad are not likely to match the whites of your printer paper for example. General viewing, by itself in isolation, the iPad will probably look just fine. But then if you really want an accurate reproduction on an iPad, a profile would help. We just.... umm - well, we haven't gotten around to it yet.