While many of the single components of an ASP.NET MVC application are easy to test (-drive) in an isolated manner (e.g. controllers), this can be hard for some others (like e.g. model binders) - and sometimes it just doesn't make much sense to test a single piece of code in isolation (e.g. for security-related issues). In such a case, some sort of integration testing has to be done, which in the context of ASP.NET MVC typically implies the usage of a browser automation framework like e.g. Selenium RC, WatiN, or the Lightweight Test Automation Framework.
