Kindle is the "killer app" that got me to buy an iPod Touch. Now that I know it is coming for iPad (even if there is some "app store approval" related delay) it helps convince me to pick up an iPad. I already know I can run the iPod/iPhone version of the Kindle app, why in the heck would Apple block Amazon from updating an already approved app so it can run on iPad? That would be pretty pathetic.
Heck, I wouldn't even be surprised if Amazon was one of the early recipients of an iPad to iron out the kinks in their Kindle app, though I must admit the priority probably went to developers whose apps generate a lot of revenue for Apple.
One of the things I don't like about the Kindle app is it sends you to Safari to buy books. I'd much rather make "in app" purchases even if there was a slight difference in price for convenience. I wonder if Bezos would be willing to let Apple take a bite out of "in app purchased" Kindle books? I bet he wouldn't mind one bit if it led to more sales. It's all about book sales. Apple is excited about yet another money machine sitting in end users' hands through which they can directly order stuff from the app store. Why would Apple consider ruffling our feathers over an unproven app like ibooks?
BTW, the Kindle app really isn't all that nice as is. It only offers 3 color schemes and a half dozen font settings. Ereader offers a lot more but ereader books are often more expensive. Now that I've mastered getting free stuff from manybooks.net over to Kindle, I find I use ereader less and less even though I like it more than the Kindle software. In the end it's the price I have to pay for books that drives which device or software I use to read them. Apple has taught the world that it's a lot easier to sell 10 billion dollars worth of music 99 cents at a time than $15.99 at a time.
Even if the only reason you're buying an iPad is for Kindle, there is no app-store related reason I can think of not to buy iPad right away. In the end, both Apple and Amazon view the hardware as an opportunity to sell you content and they don't (yet) mind one bit if both of their app stores happen to be available to you on the same device as long as in the end it means more money in their pockets.
Now if statistics were to prove that the presence of Kindle is strangling iBook sales, things could change. But not now. There are no iBook sales to strangle yet. Another question in the back of my mind is will apple block specific titles. If The Shack is available as an iBook for $12.99 and on
Kindle for $6, will Apple (or the publisher) try to insist it gets blocked? There is already some of this going on with periodicals. Many of them can only be read on Kindle hardware not on Mac, Windows or iPod/iPhone. This is the approach that will probably take shape in the future. Some titles you have "archived" on your Kindle account will be readable elsewhere but will not download to your iPad via whispersync.