Ah, what you're talking about here isn't validation, but model binding failures. You can use resources for this message.

Create a global resource class in App_GlobalResources, and set DefaultModelBinder.ResourceClassKey to the name of this class (for example, if you made "Messages.resx", then set ResourceClassKey to "Messages").

There are two strings you can override in MVC 2:

• The string value for "PropertyValueInvalid" is used when the data the user entered isn't compatible with the data type (for example, typing in "abc" for an integer field). The default message for this is: "The value '{0}' is not valid for {1}."
• The string value for "PropertyValueRequired" is used when the user did not enter any data for a field which is not nullable (for example, an integer field). The default message for this is: "A value is required."

It's important to note in the second case that, if you have the DataAnnotationsModelValidatorProvider in your validator providers list (which it is by default), then you will never see this second message. This provider sees non-optional fields and adds an implied [Required] attribute to them so that their messages will be consistent with other fields with explicit [Required] attributes and to ensure that you get client-side validation for required fields.