I saw a mention of an iphone app called MegaReader. The website is at http://www.megareader.net/ and it's in the App Store too. ( I have to say the picture of a guy reading in the car gives me a negative impression, but I think that's just marketing.) I haven't used the app yet.

I've been using Calibre to manage my kindle files. I think it lets me move things onto my iPhone, but I don't believe they're readable through the iphone kindle app. I'll check next time.