A couple more bits of code that others might find useful...  Whenever I do databinding-intensive apps (both Silverlight & WPF), I find myself writing a fair amount of boilerplate code.  If you want change notifications on your class, you need to inherit from INotifyPropertyChanged, define a PropertyChanged event, and write a couple lines of code to fire it:

    if (this.PropertyChanged!= null) {
        var args = new PropertyChangedEventArgs("SomeProperty");
        PropertyChanged(this, args);
     }

Not rocket science, but it gets old after the 10th time.  How about a ChangeableObject base class?:

    public class ChangeableObject : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        protected void NotifyPropertyChanged(String info)
        {
            if (PropertyChanged != null) {
                var args = new PropertyChangedEventArgs(info);
                PropertyChanged(this, args);
            }
        }
    }

Now you can inherit the PropertyChanged event, and simplify firing it to a single line, e.g.:

    public class Elevator : ChangeableObject
    {
        private double position = 0;
        public double Position
        {
            get { return position; }
            set { position = value; NotifyPropertyChanged("Position"); }
        }
    }

Similarly, it never ceases to annoy me that whenever I write a value converter, I'm forced to define a a ConvertBack method even though I almost never use it.  Plus, IntelliSense for implementing interfaces isn't as strong as overriding virtual methods...  So here's a trivial base class:

    public abstract class ValueConverter : IValueConverter
    {
        public abstract object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture);

        public virtual object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new Exception("unsupported");
        }
    }

Which can be used like this:

    public class BoolToVisibilityConverter : ValueConverter {
        public override object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            var v = (bool)value;
            var result = (v) ? Visibility.Visible : Visibility.Collapsed;
            return result;
        }
    }