Update: The Kindle app for iPad is now available in the Apple App Store (the free Kindle app for iPhone has been updated and says "Now supports iPad"). So the Kindle app and Apple's own iBooks app will both be available for iPad on its launch day.

The iPad has caused headaches for Amazon.com, with the online retailer forced to renegotiate with publishers who are getting the ability to set higher electronic book prices from Apple and are pushing Amazon to do the same. But for Amazon, the iPad is also a potentially significant sales channel for Kindle books. As the Wall Street Journal points out, Apple's iBooks app will not come pre-installed on the iPad, meaning people interested in reading e-books on the device can pick whatever free book app they want.

That leaves a pretty big opening for Amazon. The online retailer has already said it plans to launch a Kindle app for iPad, letting iPad users browse and buy from its large collection of e-books — 450,000 and counting —and read them on Apple's tablet computer. Apple just released its own iBooks app for iPad today. I wouldn't be surprised if Amazon follows soon, to establish itself early with iPad buyers.

As the Journal points out, Amazon has a much deeper connections to the publishing industry and a bigger selection of digital books than Apple. At the same time, the "agency" model for electronic books that Amazon is being forced to adopt by some publishers may actually boost the company's profits. The model lets publishers set higher prices on new release e-books — up to $14.99 — and gives retailers a 30 percent cut. Amazon traditionally has sold new release e-books for $9.99, reportedly at a loss.

Amazon also has free Kindle apps for iPhone, BlackBerry, PC and Mac, an indication that it sees the Kindle business as a content play across multiple devices.

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