Amazon has updated their ebook reading app for iPhone and iPad to Kindle version 3.5, and added their fancy new X-Ray feature to it, along with some improvements to manga rendering. Amazon says X-Ray lets you get "to the bones of the book" (yeah... ouch), which in less strained language means it hooks into Wikipedia and Shelfari. That means you can look up the people, places, and phrases, and descriptions from the book right inside the Kindle app. No switching required.
I mostly have comic books in Kindle, and the X-Ray button stayed grayed out for me even in the few standard books I have in my library, but if you've managed to get X-Ray working, let me know what you think of it, and how it compares to using X-Ray on Amazon's Kindle hardware proper.