If you want to use S.DS.AccountManagement (UserPrincipal and such), but you also need to update an attribute that doesn't have a built in property mapping to one of the Principal-derived classes, then you probably would want to implement your own extension class. I discuss how to do this in my Jan 2008 MSDN magazine article on S.DS.AM and it is has also been discussed in the forums several times.
If you don't want to bother with S.DS.AM or don't want to go through this extra effort, you .NET 2.0 code will still work as is.