Obsolete attribute causes property to be ignored by XmlSerialization

EDIT: After reading a MS Connect article, it appears that .Net 2.0 has a 'feature' where it makes ObsoleteAttribute equivalent to XmlIgnoreAttribute without any notification in the documentation. So I'm going to revise my answer to say that the only way to have your cake and eat it too in this instance is to follow @Will's advice and implement serialization manually. This will be your only future proof way of including Obsolete properties in your XML. It is not pretty in .Net 2.0, but .Net 3.0+ can make life easier.

From XmlSerializer:

Objects marked with the Obsolete Attribute no longer serialized In the .NET Framework 3.5 the XmlSerializer class no longer serializes objects that are marked as [Obsolete].

Another workaround is to subscribe to XmlSerializer.UnknownElement, when creating the serializer for the datatype, and then fix old data that way.

http://weblogs.asp.net/psteele/archive/2011/01/31/xml-serialization-and-the-obsolete-attribute.aspx

Maybe consider to have the method for subscribing as a static method on the class for datatype.

static void serializer_UnknownElement(object sender, XmlElementEventArgs e)
{
    if( e.Element.Name != "Hobbies")
    {
        return;
    }

    var target = (MyData) e.ObjectBeingDeserialized;
    foreach(XmlElement hobby in e.Element.ChildNodes)
    {
        target.Hobbies.Add(hobby.InnerText);
        target.HobbyData.Add(new Hobby{Name = hobby.InnerText});
    }
}