Scott Guthrie in his latest blog  post explains one of the cool new features in ASP.NET MVC 2: Model Validation. MVC already had a way of validating that worked pretty well, but with MVC 2 has improved on this with new DataAnnotation Validation attributes.

using System;
using System.ComponentModel.DataAnnotations;

namespace fekkedotcomMVC.Models
{
    public class Person
    {        
        //Example of required attributes.        
        [Required(ErrorMessage="First Name is required")]
        public string FirstName { get; set; }

        [Required(ErrorMessage = "Last Name is required")]
        public string LastName { get; set; }

        [Required(ErrorMessage = "Email is required")]
        public string Email { get; set; }
    }
}

There are also attributes for length and regular expressions. You can also add your own by extending one of the annotation classes.