Jamie Cansdale of TestDriven.NET a short message asking him for some support in the future for ASP.NET. There does not seem to be anything on his blog about ASP.NET yet, at least the search returned 0 results.
Here is a little example from NUnitAsp, which shows you how to test a label on a website:
[Test]
public void TestExample()
{
// First, instantiate "Tester" objects:
LabelTester label = new LabelTester("textLabel", CurrentWebForm);
LinkButtonTester link = new LinkButtonTester("linkButton", CurrentWebForm);
// Second, visit the page being tested:
Browser.GetPage("http://localhost/example/example.aspx");
// Third, use tester objects to test the page:
AssertEquals("Not clicked.", label.Text);
link.Click();
AssertEquals("Clicked once.", label.Text);
link.Click();
AssertEquals("Clicked twice.", label.Text);
}