UPDATE: This issue is now resolved in TestDriven.NET 3.0.2556 (http://testdriven.net/download.aspx). Thanks to Jaime Cansdale (@jcansdale) for fixing this issue so quickly.

When you try to run TestDriven.NET 3 in Visual Studio 2010 Beta 2 with xUnit.net, you might get the following exception:

------ Test started: Assembly: Mylibrary.Tests.dll ------

=== ERROR IN TEST RUNNER: xUnit.net 1.5 build 1479 ===
System.IO.FileLoadException: Could not load file or assembly 'file:///c:\users\jeff\documents\visual studio 2010\Projects\MyLibrary\Mylibrary.Tests\3rdParty\xunit\xunit.runner.tdnet.dll' or one of its dependencies. Operation is not supported. (Exception from HRESULT: 0x80131515)
File name: 'file:///c:\users\jeff\documents\visual studio 2010\Projects\MyLibrary\Mylibrary.Tests\3rdParty\xunit\xunit.runner.tdnet.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.

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It looks like you're trying to execute an xUnit.net unit test.

For xUnit 1.5 or above (recommended):
Please ensure that the directory containing your 'xunit.dll' reference also contains xUnit's
test runner files ('xunit.dll.tdnet', 'xunit.runner.tdnet.dll' etc.)

For earlier versions:
You need to install support for TestDriven.Net using xUnit's 'xunit.installer.exe' application.

You can find xUnit.net downloads and support here:
http://www.codeplex.com/xunit

To clear this issue up, you'll need to unblock the xunit reference dll's in the folder from which you're referenced them:

(do this for xunit.dll, xunit.dll.tdnet, and xunit.runner.tdnet.dll)

Afterwards, you'll be able to successfully use TD.net again without this error. (I did not have the same issue with NUnit 2.4 however)