NUnit 2.5 up to and including 2.5.4 fails to properly detect certain revisions of .NET 4.0, causing it to crash. The issue is documented in this bug report on LaunchPad. The bug has been fixed in 2.5.5 and later, so go grab the latest version of NUnit 2.5 and run your tests smoothly again, preferably on each build using Continuous Testing, of course. 
Still having trouble mocking HtmlHelper? This is an update to my previous post on mocking HtmlHelper way back when ASP.NET MVC RC1 was released. Eric notified me through a comment on the post and a question on StackOverflow that the code for ASP.NET MVC RC1 did not work with ASP.NET MVC 2. The code in this post should work with ASP.NET MVC 2 and ASP.NET MVC 3 Preview 1.
public static HtmlHelper CreateHtmlHelper(ViewDataDictionary vd)
{
Mock<ViewContext> mockViewContext = new Mock<ViewContext>(
new ControllerContext(
new Mock<HttpContextBase>().Object,
new RouteData(),
new Mock<ControllerBase>().Object),
new Mock<IView>().Object,
vd,
new TempDataDictionary(),
new Mock<TextWriter>().Object);
var mockViewDataContainer = new Mock<IViewDataContainer>();
mockViewDataContainer.Setup(v => v.ViewData)
.Returns(vd);
return new HtmlHelper(mockViewContext.Object,
mockViewDataContainer.Object);
}