As you're moving all of your Silverlight solutions into Visual Studio 2010, you might encounter an error on compile like so:

System.IO.FileLoadException: Could not load file or assembly 'file:///C:\dev\Silverlight\xxx\yyy.dll' or one of its dependencies. Operation is not supported. (Exception from HRESULT: 0x80131515)
File name: 'file:///C:\dev\Silverlight\xxx\yyy.dll' ---> System.NotSupportedException: An attempt was made to load an assembly from a network location which would have caused the assembly to be sandboxed in previous versions of the .NET Framework. This release of the .NET Framework does not enable CAS policy by default, so this load may be dangerous. If this load is not intended to sandbox the assembly, please enable the loadFromRemoteSources switch. See http://go.microsoft.com/fwlink/?LinkId=155569 for more information.

This error is due to referencing one or more assemblies that were downloaded from the web and perhaps extracted from a ZIP file - things such as third party controls. These assembly files have a "block" flag set on them for security.

There are a couple of ways around this:

1. To do a one time fix for the assemblies that are firing this error, find the assembly in Windows Explorer and right-click and select Properties. Click the "Unblock" button at the bottom right of the "General" tab.
   
   
2. UPDATE 2/11/2010: I just learned from Mark Boulter (Microsoft) that the following tweak might not be a good idea, so use at your own risk. In fact, any tweaks to devenv.exe.config are not a good idea. Anyway, I'll leave this info up in case anyone is feeling frisky.
   
   You can also permanently disable this security check in Visual Studio 2010. Open the VS2010 config file at this location:
   
    %ProgramFiles%\Microsoft Visual Studio 10.0\Common7\IDE\devenv.exe.config
   
   ...and add the following under the <CONFIGURATION> element:
   
   

    <runtime>
     <loadFromRemoteSources enabled="true" />
    </runtime>