I'm a Kindle 1 owner and was curious to see how Amazon planned to extend their platform to the iPhone. I installed the free Kindle iPhone app earlier this week and have been using it pretty much every day since.
The good news is that the app lets you sync your books with your Kindle, not just the book content itself but where you are in each one. I mention "book" a couple of times in that sentence because it's important to note that you won't get access to anything other than books through this service, which is disappointing. I subscribe to The New York Times and New Yorker magazine on my Kindle but I can't place that content on my iPhone. Yes, I know you can use the NYT's free iPhone app, but it's not the same; you have to download each article separately, which is extremely inconvenient when flying at 30K feet!
I was partway through a Stephen King book of short stories on my Kindle and I pulled it over to my iPhone. The app asked me if I'd like to jump to where I left off on my Kindle, which I did. I read a couple of stories and put it away for the night. When I went to that book on my Kindle it asked me if I'd like to jump to where I left off on my iPhone. Again, I said yes and the Kindle took me to my iPhone stopping point. Very nice.
The beauty of this is that I'll now always have all my Kindle books with me (since my iPhone is always with me). But what about those magazines? And how about all the other documents I email my Kindle account so that they're available on my device? If Amazon were smart they'd open up a service where you can keep all your documents in their cloud and access them on your Kindle or your Kindle iPhone app. I'd pay for that feature!