From the post on the MSDN blogs
Parallel Extensions will indeed be a part of the .NET Framework 4.0. Not only will it be a part of it, it will be a core part of it. In Soma’s blog post announcing the next wave, he cites the following as key focuses for the release:
• Significantly improve the core pillars of the developer experience
• Support for the latest platforms spanning the client, web, server, services and devices
• Targeted and simplified developer experiences for different application types
Parallelism is relevant to all of these. So in the .NET Framework 4.0, you’ll find the Task Parallel Library at the heart of the Framework in mscorlib.dll. You’ll find PLINQ sitting in System.Core.dll right alongside LINQ-to-Objects. And you’ll find new coordination and synchronization data types spread across various DLLs, enabling not only your applications, but the .NET Framework itself.