Lately there have been a lot of questions about when .NET 4 will be released on Windows Update (WU) and in Windows Server Update Service (WSUS). While I don’t have a specific date to share (yet…stay tuned), I can say that it will be soon (within the next couple of months). Specifically .NET Framework 4 Client Profile will be released as a recommended update on Vista and Windows 7 (a recommended update will be automatically installed depending on the Automatic Update settings of the machine). Client Profile will also be released as an optional update for XP. With XP you will have to manually run WU and select the .NET Framework 4 Client Profile. Windows 2003, Windows 2008 and Windows 2008 R2 will see .NET Framework 4 as an optional update.
IT administrators using WSUS will have a slightly different experience. .NET Framework 4 will be optional on all the above OSs, while .NET Framework 4 Client Profile will be optional on XP and recommended on Vista and Windows 7.
There is an article that describes how to temporarily block the installation of the Microsoft .NET Framework 4 Client Profile. You might have to do this in an environment in which the Automatic Updates feature is enabled on a computer that is running Windows Vista or on a computer that is running Windows 7. See KB http://support.microsoft.com/kb/982320
Important: We recommend that you do not block the installation of the .NET Framework 4. However, if you have a compelling business reason to do this temporarily in environments where the Automatic Updates feature is enabled, you can modify the registry to temporarily block the installation of the .NET Framework 4 Client Profile.