SharePoint 2010 introduces new ways of developing applications for the SharePoint platform. With SharePoint 2010, you can build multi-tenant, hosted applications on an infrastructure that is scalable, secure, and stable. You can create powerful, data-driven solutions using SharePoint lists or external data. You can create client-side functionality that takes advantage of modern browser capabilities and Silverlight to provide a rich user experience. These new features, operational models, and development tools make SharePoint a more complete application platform, and it also introduces new design and development decisions for customers. This guidance helps customers understand the decision points, tradeoffs, and performance implications that the new functionality introduces; it also helps customers learn how to take best advantage of the new capabilities that SharePoint 2010 provides.
SharePoint Team Blog posting: http://blogs.msdn.com/b/sharepoint/archive/2010/06/30/developing-applications-for-sharepoint-2010.aspx
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ISV Architect Evangelist