Amazon has just released Amazon Kindle for iPhone. Since its initial release in 2007, the Kindle has been a hot seller, often having buyers on a waiting list to get one. The Kindle 2 was recently released and, by most accounts, it improves on its popular predecessor. For Amazon, this isn't about the hardware, though, it's about the content, and the company just took its first step into the content-only sales model for Kindle books.At $359 for the Kindle 2, I suspect Amazon is making a profit off the device, so this isn't like the printer/ink refill model where printers may sell at a loss but the profits on ink refills more than make up for it in relatively short order. However, Amazon is about content, and selling the "ink refills" to Kindle users is where it really makes its money. By creating this software for the iPhone and iPod Touch, it just opened its customer base by well over 20 million users. Being free, I'd bet this will be the top application downloaded for a few days and Kindle book sales will go through the roof as people try it out. If you already have a Kindle, you will be able to use the WhisperSync feature to keep your Kindle and iPhone in sync, so you can read your Kindle at night before going to sleep and then pick up where you left off on your iPhone while sitting on the train in your morning commute. I've not seen further details on which platforms Amazon will target with this, but Windows Mobile, Android, and Palm WebOS (Pre) should all be powerful enough to run this, which will open up the user base by tens of millions more. Netbooks aren't quite as portable as the Kindle, but for some students on a budget, $359 for the Kindle 2 or the same dollars for a netbook is a pretty easy decision when the latter will aid in getting schoolwork done and keeping their Facebook profile up to date. Hopefully, Amazon recognizes this and will be releasing software for the PC market. Amazon is becoming a content giant. It already has a successful MP3 store and I am always renting TV shows or movies from it via TiVo. Now in 18 months, when people think of ebooks, Amazon's Kindle service comes to mind.