MVC Form Validation on Multiple Fields

I actually ended up implementing a custom ValidationAttribute to solve this, using the same type of logic presented in CompareAttribute that allows you to use reflection to evaluate the values of other properties. This allowed me to implement this at the property level instead of the model level and also allows for client side validation via unobtrusive javascript:

public class MultiFieldRequiredAttribute : ValidationAttribute, IClientValidatable
    {
        private readonly string[] _fields;

        public MultiFieldRequiredAttribute(string[] fields)
        {
            _fields = fields;
        }

        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            foreach (string field in _fields)
            {
                PropertyInfo property = validationContext.ObjectType.GetProperty(field);
                if (property == null)
                    return new ValidationResult(string.Format("Property '{0}' is undefined.", field));

                var fieldValue = property.GetValue(validationContext.ObjectInstance, null);

                if (fieldValue == null || String.IsNullOrEmpty(fieldValue.ToString()))
                    return new ValidationResult(this.FormatErrorMessage(validationContext.DisplayName));
            }

            return null;
        }

        public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
        {
            yield return new ModelClientValidationRule
            {
                ErrorMessage = this.ErrorMessage,
                ValidationType = "multifield"
            };
        }
    }