I knew Apple would approve it. They wouldn't risk the negative publicity they would garner by not approving it. Apple doesn't really care if you buy iBooks or not, as they make the most money off the hardware. And if more people buy the iPad just because of the Kindle app, then Apple profits from it. This is actually great news for Amazon as well though, because if you already have a small kindle library like I have, why would you give it up to go to iBooks that has less books?
This app single-handedly makes the iPad the most versatile ebook reader on the market. Whether or not it will kill the iBook store remains to be seen. People who have no allegiance to either store will probably want to go with Apple as they made the device.