Download and more reference material here:

http://msdn.microsoft.com/en-us/vstudio/async.aspx

Great crash course article from Alexandra Rusina here:

http://blogs.msdn.com/b/csharpfaq/archive/2010/10/28/async.aspx

Executive summary:

Basically you get two new powerful C# language keywords, await and async.  Instead of having to write callback methods for multithreaded code, you can now decorate a method signature with async and call the method with await. You write the code as if it were a synchronous block of code and let the runtime perform the heavy lifting.  Very cool and really streamlined.  I wonder what debugging will look like. 

Configuration Error

Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.
Parser Error Message: Unrecognized attribute 'multipleSiteBindingsEnabled'. Note that attribute names are case-sensitive.
Source Error:

Line 105:  -->
Line 106:	<system.serviceModel>
Line 107:		<serviceHostingEnvironment multipleSiteBindingsEnabled="true" />
Line 108:  <services>
Line 109:			<service behaviorConfiguration="RFID.Server.WCF.Service1Behavior" 
name="RFID.Server.WCF.Service1">

Source File: C:\inetpub\web\services.test.\RFIDServer\web.config    Line: 107

---

Version Information: Microsoft .NET Framework Version:4.0.21006; ASP.NET Version:4.0.21006.1

---

Received the above error message when executing a WCF service on a test server.  The WCF service was compiled with .NET  4.0 RC and the test server only had .NET 4.0 Beta 2 installed.  The fix was easy.  Install .NET 4.0 RC full runtime on the test server and it worked.

The “Microsoft.WebApplication.Build.Tasks.GetSilverlightItemsFromProperty” task could not be loaded from the assembly … Microsoft.WebApplication.Build.Tasks.dll. … The assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded.  Confirm that the <UsingTaskk> declaration is correct, and that the assembly and all its dependencies are available.

So I am sure there is a way to tweak / edit the solution files and .csproj files to fix this.  I had not gotten into much coding with the Silverlight side of this solution.  I had been working on the Model and WCF stuff (targeting .NET 4.0) so I encountered this when I started on some ViewModel code. 

Clearly this is because the WebApplication project created for the Silverlight 3 application is trying to build with the .NET 4.0 version of the MSBuild environment and Blend 3 wants the 3.5/2.0 version.

The simplest way to fix this is to create the Silverlight + Website project from Blend 3.  Then Add the projects to your VS 2010 B2 solution.  Then everything works.