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There are recent customer questions about .NET Framework support in SharePoint. For example, there is this question in the SharePoint 2010 forums, as well as several customer questions at SharePoint booth at TechEd.
SharePoint 2010 does not support access from .NET Framework 4 or being run in a .NET Framework 4 app domain. If you must support the .NET Framework 4, we recommend that you develop a web service that uses the .NET Framework 4 and then connect to it from SharePoint. All server-side customizations directly for SharePoint 2010 still need to be either the .NET Framework 3.5 or the .NET Framework 3.5 SP1.
For more information, see the following MSDN Library topics.
However, you can use Silverlight 4 and Silverlight 5 against the SharePoint client object model. For more information, see the following MSDN Library topics.