悲しいけどこれ、仕様なのよね。。（Silverlightの愚痴エントリ）

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Collections.Specialized;
using System.Windows.Controls;

namespace SharpLab.CredentialsLockerSilverlight.ViewModels {
	public class ReadOnlyObservableTabItemCollection<TViewModel, TViewModelCollection> : ICollection<TabItem>, INotifyCollectionChanged where TViewModelCollection : ICollection<TViewModel>, INotifyCollectionChanged {

		private TViewModelCollection _viewModels;
		private ObservableCollection<TabItem> _tabItems;
		private Func<TViewModel, TabItem> _converter;

		public ReadOnlyObservableTabItemCollection(TViewModelCollection viewModels, Func<TViewModel, TabItem> converter) {
			_converter = converter;
			_viewModels = viewModels;
			_tabItems = new ObservableCollection<TabItem>();

			foreach (var item in _viewModels) {
				_tabItems.Add(_converter(item));
			}

			_viewModels.CollectionChanged += new NotifyCollectionChangedEventHandler(_viewModels_CollectionChanged);

			_tabItems.CollectionChanged += new NotifyCollectionChangedEventHandler(_tabItems_CollectionChanged);
		}

		void _viewModels_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e) {
			switch (e.Action) {
				case NotifyCollectionChangedAction.Add:
					_tabItems.Insert(e.NewStartingIndex, _converter((TViewModel)e.NewItems[0]));
					break;
				case NotifyCollectionChangedAction.Remove:
					_tabItems.RemoveAt(e.OldStartingIndex);
					break;
				case NotifyCollectionChangedAction.Replace:
					_tabItems.RemoveAt(e.OldStartingIndex);
					_tabItems.Insert(e.NewStartingIndex, _converter((TViewModel)e.NewItems[0]));
					break;
				case NotifyCollectionChangedAction.Reset:
					_tabItems.Clear();
					foreach (var item in _viewModels) {
						_tabItems.Add(_converter(item));
					}
					break;
			}
		}

		void _tabItems_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e) {
			if (CollectionChanged != null) {
				CollectionChanged(this, e);
			}
		}

		public event NotifyCollectionChangedEventHandler CollectionChanged;

		#region ICollection<TabItem> メンバ

		public void Add(TabItem item) {
			throw new NotSupportedException();
		}

		public void Clear() {
			throw new NotSupportedException();
		}

		public bool Contains(TabItem item) {
			throw new NotImplementedException();
		}

		public void CopyTo(TabItem[] array, int arrayIndex) {
			throw new NotSupportedException();
		}

		public int Count {
			get {
				throw new NotImplementedException();
			}
		}

		public bool IsReadOnly {
			get {
				return true;
			}
		}

		public bool Remove(TabItem item) {
			throw new NotSupportedException();
		}

		#endregion

		#region IEnumerable<TabItem> メンバ

		public IEnumerator<TabItem> GetEnumerator() {
			foreach (var item in _tabItems) {
				yield return item;
			}
		}

		#endregion

		#region IEnumerable メンバ

		System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() {
			return GetEnumerator();
		}

		#endregion
	}
}

using System;
using System.Collections.ObjectModel;
using System.Windows.Controls;
using System.Windows.Data;
using SharpLab.CredentialsLockerSilverlight.ViewModels;

namespace SharpLab.CredentialsLockerSilverlight.Views.Converter {
	public class CardVMCol2TabItemColConverter : IValueConverter{
		#region IValueConverter メンバ

		public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) {
			return new ReadOnlyObservableTabItemCollection<CardViewModel, ObservableCollection<CardViewModel>>((ObservableCollection<CardViewModel>)value,
				(each) => {
					var newTabItem = new TabItem() {
						DataContext = each,
						Header = new CardTabHeaderView(),
						Content = new SharpLab.CredentialsLockerSilverlight.Views.CardView()
					};
					return newTabItem;

				});
		}

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

		#endregion
	}
}

 ItemsSource="{Binding Path=CardViewModels, Converter={StaticResource CardVMCol2TabItemColConverter}}"