If you want to share data amongst all instances of a class you will naturally create some "static" variables. This is fine, except when using the binding engine within Silverlight, as it relies on "PropertyChanged" events to notify the UI of a change to a property, but you can't raise an instance event from within a static property. The way I solved this was to create a static event called "StaticPropertyChanged" which the static properties raise whenever they are changed. Within the constructor of the class I attach to this event (i.e. each instance attaches to the static event) and re-raise the event through the PropertyChanged event for the current instance. In effect, every instance of the class will raise a PropertyChanged event whenever any of the static data is changed. The code is as follows:
public abstract class ViewModelBase : NotificationObject
    {
	//use a shared event so that any instance can propogate the event to all instances
	private static event Action<string> _staticPropertyChanged;

	protected static void OnStaticPropertyChanged(string propertyName)
	{
		if (_staticPropertyChanged != null)
		_staticPropertyChanged(propertyName);
	}         

	public ViewModelBase()
	{
		//when a new instance is created, we subscribe it to shared Busy Property Changed event
		_staticPropertyChanged += ViewModelBase__staticPropertyChanged;
	}

	/// <summary>
	/// Within each instance, handle the shared staticPropertyChanged event by raising the instance version of RaisePropertyChanged event
	/// </summary>
	void ViewModelBase__staticPropertyChanged(string propertyName)
	{
		RaisePropertyChanged(propertyName);
	}


	//example
	private static Dictionary<long, string> _SomeSharedLookup;
	public static Dictionary<long, string> SomeSharedLookup
	{
		get { return ViewModelBase._SomeSharedLookup; }
		set { ViewModelBase._SomeSharedLookup = value; OnStaticPropertyChanged("SomeSharedLookup"); }
	}
}