I have recently uploaded TestDriven.NET 2.5 Beta. This version now supports all versions of Visual Studio .NET including the Express SKU. To activate TestDriven.NET when using an Express edition simply ensure that the 'Properties' tool window is visible and click on any node in the 'Solution Explorer'. You should see a 'TestDriven' property which shows the version of TestDriven.NET you are using when the extension is active.
You can download this version from here.
TestDriven.NET has been designed to work with C#, VB, C++ and J# Express. Visual Web Developer Express is not yet supported. I'm hoping to add support for this and Visual Studio 2005 'Web Site' projects in a future version.