PayPal has announced that it's bringing a new iOS SDK and some new API tools to SXSW next week. As you might expect, the API will allow apps to use PayPal's tools to integrate payment information directly rather than having to go through a separate authorization page. As you can see on the official website, devs will be able to simply use a few method calls to set up PayPal payments, and get proof back that payment has taken place.
So developers will have a lot of new tools to work with PayPal's payment system. It's unclear, however, what Apple thinks of this development -- it seems to me that paying through PayPal from an iOS app gets around Apple's requirement of taking a cut from everything sold on the App Store. But that's something that Apple and PayPal will need to work out. If you're a developer who wants to dive in to what's available here, you can download the PayPal iOS SDK for free right now.
Update: PayPal president David Marcus just contacted me via Twitter to say that this SDK is meant for service transactions, not digital sales, so it doesn't fall under Apple's rules. In other words, the tools here are to be used for selling things outside the iPhone with Apple's device, not selling iPhone content or other items directly.