I know this is an old question, but if you encounter this problem in MVC 3 then you can decorate your ActionMethod with [ValidateInput(false)] and just switch off request validation for a single ActionMethod, which is handy. And you don't need to make any changes to the web.config file, so you can still use the .NET 4 request validation everywhere else.
e.g.
[ValidateInput(false)]
public ActionMethod Edit(int id, string value)
{
// Do your own checking of value since it could contain XSS stuff!
return View();
}