Normally I would do this:

public class DBFactory
{
        public UserDAO GetUserDao()
        {
               return new UserDao();     
        }

}

Where UserDao being the concrete implementation of IUserDao.

So now my code will be littered with:

DBFactory factory = new DBFactory();

IUserDao userDao = factory.GetUserDao();

User user = userDao.GetById(1);

Now if I wanted to swap implementaitons, I would have to go to my DBFactory and change my code to call a different implementation.

Now if I used NINject, I would bind the specific implementation on application startup, or via a config file. (or bind based on specific parameters etc. etc.).

Is that all there is too it? Or is there more?

(reason I am asking if I want to know how it will help me here: http://stackoverflow.com/questions/1930328/help-designing-a-order-manager-class)