Out of curiosity, I installed the Kindle iPhone app this morning to poke around with it, and see what a "premium" book reader for the iPhone would be like. It's no surprise that the iPhone screen is a suboptimal display format for book reading, there's a reason the Kindle is the size it size... On the other hand, I didn't need to shell out many hundreds of dollars in new hardware costs (well, beyond the several hundreds I had already spent for the iPhone.)
It was simple enough to install the application and link it to my Amazon account via my email address. Once I did, the iPhone immediately showed up as a downloadable device when I went to the Kindle pages on Amazon. I bought Denis Leary's Why We Suck, and when I return to my iPhone, it was already downloaded and ready.
That being said, there's one reason I will absolutely not be using the Kindle software to read a lot of books. Right justification. You hearing me there, Amazon? Nothing makes text harder to read in a narrow column than right justification. Give me some ragged right borders, so I can keep track of what fracking line I'm on, and I'll be a much happier camper.