Let me start off by saying that this is not an iPhone vs. BlackBerry post. It is an Amazon Kindle post.
There are, undoubtedly, iPhone users that will never buy an Amazon Kindle, however, might possibly by an ebook for the Kindle since there is a Kindle App for the iPhone. Think about it, Amazon will make a lot of money selling the Kindle. They, will, however, over time make a lot more money selling the books.
There are over 50 million BlackBerry users out there. Many of them will never buy a Kindle either. If there were an Amazon Kindle App for the BlackBerry, guess what a few of them might buy?