PayPal X Platform is a new open source toolkit for Google App Engine that provides an easy way to integrate Java apps running on App Engine with new PayPal's Adaptive Payments API. Currently this's a Java toolkit explicitly for App Engine Java, but a Python version will be coming out soon.
"Getting started with App Engine toolkit is easy. You can either checkout source code from svn and import it into your Eclipse project or simply download prebuilt jar file and include it in your app's class path (/WEB-INF/lib). Similar to other APIs that you might have used, you would need PayPal API Credentials to authenticate your API requests. With toolkit you simply create an "APICredential" object from one of your app initialization methods (in most cases from your Servlet init() method) and load API Credentials that you've obtained from PayPal Sandbox," explains Google.