As we announced at MIX 2010, .NET 4 will be available in Windows Azure within 90 days of .NET 4 RTM. As part of our preparation for that, the latest operating system build available in Windows Azure contains the .NET 4 RC. Although you cannot use this build to run .NET 4 applications, please let us know if having .NET 4 RC in the build has any effect on your existing applications.

One known effect you may see if you’re consuming generic ASP.NET performance counters is that they will report data only on .NET 4 applications. You can instead use the versioned performance counters as documented in KB article 2022138.

As always, you can choose which build of the operating system your application will run on in Windows Azure. See the MSDN documentation for details.