Believe it or not, my iPad has made me a fan of the Kindle. I don't have a kindle, and until their prices drop considerably, I won't have one anytime soon. However, I do have an iPhone and like to read books on both my iPad (when home) and my iPhone (when away, and sometimes at night in bed).
The Kindle app for the iPad has some things not as good as iBooks and some things better. It's killer feature is the ability to sync your place between reading on multiple devices. This ability is coming to the iPad/iPhone this summer when iPhone OS 4.0 is released.
What isn't likely to ever come, is an eInk reader that works with the Apple book store. There is no doubt that for reading outside in full sun, an eInk reader has it all over any lcd device. So why wouldn't I stick with getting my books from the store that has the most options for me reading them? Who knows, I may be picking up a kindle at a garage sale some 5 years from now. In the mean time, all the advantages of the iPad for reading (and there are many), I also get via the kindle app on the iPad.
I could say the same thing for Barnes and noble as soon as they release their book app for the iPad. You can read such books on a Nook, an iPhone or an iPad.
Apple may have succeeded in erasing price as a differentiator by agreeing with the publishers to go with the agency model for ebooks. A book sold under the agency pricing model is going to cost the same, no matter where you buy it. But if you buy from Amazon (or Barnes and Noble), you actually have more options for reading the book -- even on Apple's devices.
FYI - I have had my iPad for more than a week and am on my third book. It's a GREAT reader indoors (where I do most all of my reading). By adjusting the brightness and font size....I find that I can read for hours with no eye strain. Of course it's also great for surfing the web, watching videos, playing games
Lee