Android users have been able to take advantage of in-app purchases via PayPal, Glu Mobile or the Android Market's native tools for a while now, and Amazon is naturally going to be bringing the same capability to the Android-based Kindle Fire tablet. The online retailer is beginning a developer beta program for in-app purchasing even before the tablet goes on sale next month. Developers can request access to the program today.
The Kindle Fire uses a completely skinned/forked version of Android Gingerbread, and has precisely zero of Google's proprietary software installed. That means that since it swaps out the Android Market for the Amazon App Store, developers selling apps for the Fire don't have access to Google Mobile Services. Amazon's solution will use their own extensive back-end to allow games and marketplaces to sell digital items and upgrades directly within apps.
The pre-registration form for the beta program indicates that Amazon is targeting subscription apps and one-time purchases. The form also asks applicants if they've used Google Checkout, PayPal or the iOS App Store for in-app purchases before, but that's likely just for research. I can't imagine Amazon using anything except its own checkout process for purchases on the Fire.
The Kindle Fire is currently set for a November 15th release. Amazon has already racked up more than 250,000 preorder sales.