<Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,12">
<ListBox>
<ListBoxItem HorizontalContentAlignment="Stretch">
<toolkit:PhoneTextBox x:Name="PinNameTextBox" Hint="{Binding Path=LocalizedResources.Untitled, Source={StaticResource LocalizedStrings}}" Text="{Binding Name, Mode=TwoWay}" />
</ListBoxItem>
<ListBoxItem HorizontalContentAlignment="Stretch">
<TextBox x:Name="PinContentTextBox" Text="{Binding Text, Mode=TwoWay}" AcceptsReturn="True" TextWrapping="Wrap" InputScope="Text" />
</ListBoxItem>
</ListBox>
</Grid>
I have also tried the ScrollViewer but it does not work.
I think this was already the same case with WP7; from what I've heard it is a platform restriction that causes UIElements like e.g. TextBoxes to be clipped if they exceed 2048px.
You could try to create an own TextBox control with a similar approach to what was done here, though. Hope that helps.