gistfile1.cs

        public IEnumerable SelectedItems
        {
            get
            {
                return this.selectedItems;
            }
            set
            {
                this.selectedItems = value;
                this.RaisePropertyChanged("SelectedItems");//比較方法がわからないので無条件に発生

                StatusViewModel selectedStatus = this.SelectedItem as StatusViewModel;
                if (selectedStatus != null)
                {
                    //ここ
                    this.Items.Cast<object>().AsParallel().ForAll(item =>
                    {
                        StatusViewModel statusItem = item as StatusViewModel;
                        if (statusItem != null)
                        {
                            statusItem.IsSelectedItemsReply = statusItem.ID == selectedStatus.InReplyToStatusId;
                            statusItem.IsSelectedUser = statusItem.User.ID == selectedStatus.User.ID;
                            statusItem.IsSelectedItemsReplyUser = selectedStatus.Text.Contains("@" + statusItem.User.ScreenName);
                        }
                    });
                }
            }
        }