Kindle Cloud Reader is a Chrome and Safari friendly web app where you can read and (more importantly) buy Kindle ebooks. It doesn't work on the iPhone yet, but if you download Chrome or Safari you can try it on your PC.
But is it a better option than the existing apps?
Well, it is platform independent, and you can use it to read ebooks offline. It has the same basic features as the other apps, but it's missing the search function as well as the brightness pseudo-function. I feel that search is rather important, myself. It's also missing the more sophisticated display options found in the desktop app.
Most importantly, the KCR also lacks the one best feature of the iOS apps. It can't play the audio or video in embedded ebooks anymore. It's a small thing, but it's enough for me to keep the old app on my iPad.
I would guess that this is going to be Amazon's primary app for now on, which means that the missing features will likely be added eventually. (Support for additional browsers is promised on the landing page.) At some point I could see myself switching over, just not yet. I like the idea of having the app in the browser as opposed to a stand alone app, I really do. But the features have to be there before I'll commit.