With the release of Visual Studio 2010, we also released the .NET 4 reference sources. The following binaries represent the published .NET 4 managed reference source:

microsoft.visualbasic.dll
mscorlib.dll
PresentationBuildTasks.dll
PresentationCore.dll
PresentationFramework.Aero.dll
PresentationFramework.Classic.dll
PresentationFramework.Luna.dll
PresentationFramework.dll
PresentationFramework.Royale.dll
PresentationHost_v0400.dll
System.AddIn.Contract.dll
System.AddIn.dll
System.Core.dll
System.Data.DataSetExtensions.dll
System.Data.Entity.Design.dll
System.Data.Entity.dll
System.Data.Linq.dll
System.Data.dll
System.Data.Services.Client.dll
System.Data.Services.Design.dll
System.Data.Services.dll
System.Data.SqlXml.dll
System.Drawing.dll
System.Management.dll
System.dll
System.Printing.dll
System.Security.dll
System.Web.DynamicData.dll
System.Web.Entity.Design.dll
System.Web.Entity.dll
System.Web.Extensions.dll
System.Web.Mobile.dll
System.Web.dll
System.Web.Routing.dll
System.Windows.Forms.dll
System.Xml.Linq.dll
System.Xml.dll
UIAutomationClient.dll
UIAutomationClientsideProviders.dll
WindowsBase.dll
ComSvcConfig.dll
SMDiagnostics.dll
System.IdentityModel.dll
System.IdentityModel.Selectors.dll
System.IO.Log.dll
System.Runtime.Serialization.dll
System.ServiceModel.dll
System.ServiceModel.WasHosting.dll
System.ServiceModel.Web.dll
System.Transactions.dll
System.Web.Services.dll
System.Workflow.Activities.dll
System.Workflow.ComponentModel.dll
System.Workflow.Runtime.dll
System.WorkflowServices.dll
WsatConfig.dll
System.Activities.dll
System.Activities.core.presentation.dll
System.Activities.presentation.dll
System.ServiceModel.Activities.dll
System.ServiceModel.Routing.dll
System.ServiceModel.Discovery.dll
System.ServiceModel.Channels.dll
System.Runtime.DurableInstancing.dll
System.Activities.DurableInstancing.dll
Microsoft.VisualBasic.Activities.Compiler.dll
Microsoft.VisualBasic.Activities.CompilerUI.dll  
System.Xaml.Hosting.dll
XamlBuildTask.dll
System.Messaging.dll
System.Runtime.Remoting.dll
System.Runtime.Serialization.Formatter.Soap.dll
System.ServiceModel.Activation.dll
System.ServiceModel.Discovery.dll

 The .NET 4 file version for this release is 4.0.30319.1.

And for those of you that need offline availability to step into the .NET Reference source with Visual Studio 2010, here is where you can find the offline packages.

http://referencesource.microsoft.com/netframework.aspx

 Instructions for setting up Visual Studio for reference source stepping can be found here:

http://blogs.msdn.com/b/sburke/archive/2008/01/16/configuring-visual-studio-to-debug-net-framework-source-code.aspx

Note, you don't have to install any hotfix's to get started with Visual Studio 2010 source stepping. Enjoy!