This post on GigaOm by Narendra Rocherolle talks exactly about what Apple needs to do soon with its SDK and the App Store. I've been playing around with a lot of apps, as well as my own ideas for apps, and this is something I've also been thinking is sorely needed for several reasons:
1) Getting small developers to develop a billing infrastructure for their apps is unrealistic. Today, and for the foreseeable future, the majority of apps will come from small developers
2) There are only 2 ways to make money off apps today: an up-front payment or advertising. A payments API would allow subscriptions and ad-hoc charging for services/content, giving developers much more flexibility in how they build and charge for their apps.
3) Even larger ISVs that can afford to build out a way to charge today face uncertainty around whether or not Apple will allow their app into the App Store. To date, I have not seen an app that charges for service/content post purchase and, frankly, I wouldn't be surprised if Apple shuts down those who do (which brings up a bunch of other issues...).
Apple should do this ASAP if they want to turn the iPhone a market-leading platform.