As the whole unobstrusive client validation from MVC 3 rely on the
model and I don’t feel like putting a ConfirmPassword property on my
model,what should I do?

完全同意你的意见。这就是为什么你应该使用视图模型。然后在您的视图模型(专门针对给定视图的要求设计的类)中，您可以使用[Compare]属性：

public class Registerviewmodel
{
    [required]
    public string Username { get; set; }

    [required]
    public string Password { get; set; }

    [Compare("Password",ErrorMessage = "Confirm password doesn't match,Type again !")]
    public string ConfirmPassword { get; set; }
}

然后让您的控制器操作采用此视图模型

[HttpPost]
public ActionResult Register(Registerviewmodel model)
{
    if (!ModelState.IsValid)
    {
        return View(model);
    }

    // TODO: Map the view model to a domain model and pass to a repository
    // Personally I use and like AutoMapper very much (http://automapper.codeplex.com)

    return RedirectToAction("Success");
}