How to design Date-of-Birth in DB and ORM for mix of known and unknown date parts

Note up front, my question turns out to be similar to SO question 1668172.

This is a design question that surely must have popped up for others before, yet I couldn't find an answer that fits my situation. I want to record date-of-birth in my application, with several 'levels' of information:

• NULL value, i.e. DoB is unkown
• 1950-??-?? Only the DoB year value is known, date/month aren't
• ????-11-23 Just a month, day, or combination of the two, but without a year
• 1950-11-23 Full DoB is known

The technologies I'm using for my app are as follows:

• Asp.NET 4 (C#), probably with MVC
• Some ORM solution, probably Linq-to-sql or NHibernate's
• MSSQL Server 2008, at first just Express edition

Possibilities for the SQL bit that crossed my mind so far:

• 1) Use one nullable varchar column e.g. 1950-11-23, and replace unkowns with 'X's, e.g. XXXX-11-23 or 1950-XX-XX
• 2) Use three nullable int columns e.g. 1950, 11, and 23
• 3) Use an INT column for year, plus a datetime column for full known DoBs

For the C# end of this problem I merely got to these two options:

• A) Use a string property to represent DoB, convert only for view purposes.
• B) Use a custom(?) struct or class for DoB with three nullable integers
• C) Use a nullable DateTime alongside a nullable integer for year

The solutions seem to form matched pairs at 1A, 2B or 3C. Of course 1A isn't a nice solution, but it does set a baseline.

Any tips and links are highly appreciated. Well, if they're related, anyhow :)

Edit, about the answers: I marked one answer as accepted, because I think it will work for me. It's worth looking at the other answers too though, if you've stumbled here with the same question.