I didn't like any of the solutions I found so I kept poking at possibilities until I came up with one I do like. I added a regular expression validator utilizing the regular expression from this article: http://answers.oreilly.com/topic/226-how-to-validate-traditional-date-formats-with-regular-expressions/
[Required(ErrorMessage = "Birthdate is required. [MM/DD/YYYY]")]
[RegularExpression(@"^([1-9]|0[1-9]|1[0-2])[- / .]([1-9]|0[1-9]|1[0-9]|2[0-9]|3[0-1])[- / .](1[9][0-9][0-9]|2[0][0-9][0-9])$", ErrorMessage = "Birthdate must be in MM/DD/YYYY format.")]
public Nullable<DateTime> Birthdate { get; set; }
The result is, if the field is blank I get the required error message and if anything is in the field, but it is not a valid date, I get the regular expression message.
I might add that it seems very silly that [DataType] doesn't accept an error message. I tried exactly like the original author of this thread. That would have been logical and intuitive.