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Microsoft patterns and practices have released a guide: Developing Applications for SharePoint 2010
In the guide:
"Application Foundations for SharePoint 2010" describes approaches you can use to address testing, flexibility, configuration, logging and exception handling, and maintainability.
"Execution Models in SharePoint 2010" provides technical insights into the mechanics of the full-trust execution environment, the sandbox execution environment, and various hybrid approaches to executing code in SharePoint applications.
"Data Models in SharePoint 2010" explains new list and external data functionality and data access techniques, key design decision points that can help you to choose between standard SharePoint lists and external lists, and techniques and patterns to address large lists and list aggregation.
"Client Models in SharePoint 2010" provides guidance on how to best use the new client-side functionality to access data and build richer client experiences with Silverlight and Ajax.
Each section also contains a set of how-to topics. These explain how to perform specific tasks that the team found challenging to discover.