Wonder no more: Amazon is building a Kindle app for the iPad (and other tablets) as evidenced by this page on its website today. The app looks to be similar in functionality to the iPhone Kindle app, with Whispersync technology and full color and would allow you access to all of the books you’ve already purchased for Kindle.
The only question that remains: Will Apple let it onto the iPad App Store? There is already a Kindle App in the App Store, so this would only be an “upgrade”.
On one hand, people who have Kindles or who have purchased Amazon eBooks would be more likely to buy the iPad. However, if a beautiful, color Kindle.app is in the App Store on day one, which allows you to read books from other devices including your Mac, it may be hard for the iBookstore to gain any footing in the market.